Introduction
A sinkhole is a natural topographic depression formed when surface material subsides or collapses into voids in the subsurface, and it often marks a direct connection between surface and underground systems. Various terms are used to emphasize different aspects of these features: doline or shakehole for enclosed depressions; ponor, swallow hole or swallet for openings that funnel surface water into subterranean conduits; and sink or stream sink for sites that drain surface flow into underlying strata. A cenote denotes a specific variety in which the depression exposes the groundwater table or a submerged cave, providing direct access to aquifers. Most sinkholes develop in karst environments where dissolution of carbonate bedrock (e.g., limestone, dolomite) creates cavities and conduits that later fail either by structural collapse or by gradual removal of supporting sediment (suffosion). Morphologies are commonly near‑circular and vary from shallow, soil‑lined bowls to deep, rock‑edged chasms, with dimensions typically ranging from tens to hundreds of metres. Development may be slow and progressive as bedrock dissolves and ground settles, or abrupt when subsurface support is suddenly lost. Functionally, sinkholes are important hydrologic connectors that influence recharge, local drainage patterns and contaminant pathways, and they occur wherever susceptible lithologies and hydrological conditions prevail worldwide, exemplified by features such as the Red Lake sinkhole in Croatia.
Formation of evaporite-related sinkholes is well illustrated by contrasting modern cases at the Dead Sea and Chinchón, Spain. At the Dead Sea, a progressive lowering of the lake surface reverses or reduces hydraulic heads in adjacent aquifers, allowing low‑salinity groundwater to move laterally into formerly saline, evaporite‑rich sediments. This advective intrusion dissolves subsurface halite and other evaporites, creating voids that, once sufficiently large or undermined, collapse to produce sinkholes along the retreating shoreline. By contrast, the Chinchón event exemplifies a continental collapse sinkhole, in which sudden surface subsidence occurs into pre‑existing underground cavities that may have originated from natural dissolution of soluble strata or from anthropogenic removal of subsurface material.
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The spatial and temporal distribution of such sinkholes is governed by coupled hydrological and geochemical controls. Falling water bodies change hydraulic gradients and promote inflow of fresher water into evaporite layers; the rate and extent of dissolution reflect water chemistry, flow velocity, and the thickness and lateral continuity of subsurface salt beds. Consequently, sinkhole occurrence is often clustered where these conditions coincide. Morphologically, dissolution‑driven collapses tend to be abrupt once critical void dimensions are reached, producing steep‑walled pits of highly variable diameter and depth. As the hydrogeologic regime evolves, collapse zones can migrate inland from former shorelines, creating acute hazards to roads, buildings, agricultural land and other infrastructure.
Given these dynamics, effective hazard management requires integrated subsurface and hydrological assessment coupled to land‑use controls. Targeted geophysical surveys and borehole data help delineate the geometry and continuity of evaporite horizons; groundwater monitoring can detect freshwater intrusion and shifting hydraulic heads; and systematic mapping of sinkhole occurrence over time defines expanding risk zones. Where exposure cannot be avoided, engineering measures or restrictions on development should be employed to reduce the likelihood and consequences of sudden collapse.
Natural processes forming sinkholes
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Sinkholes are surface depressions or collapses that result from the removal of subsurface support; they may capture surface runoff or form in apparently dry, elevated terrain wherever subsurface conditions permit void creation. The dominant mechanisms are: chemical dissolution of slightly soluble bedrock (most commonly carbonates such as limestone, but also salt and gypsum); mechanical failure of cavern roofs; and hydrologic changes—for example, a falling water table—that strip away buoyant or hydrostatic support and precipitate collapse.
A significant process in many terrains is suffosion, in which groundwater progressively removes the carbonate cement that bonds granular material (e.g., in sandstones). As the cement dissolves, grains become mobilized and are washed into deeper conduits, producing subsurface cavities that may ultimately lead to gradual subsidence or sudden collapse at the surface. Although most frequent in soluble lithologies, sinkholes can also develop in less-soluble rocks (sandstone, quartzite) wherever particle or cement support is lost.
Progressive rock dissolution commonly enlarges underground voids and caverns while leaving a coherent surficial layer intact; when that layer fails, collapse can be abrupt and dramatic. Some large sinkholes expose active cave passages or flowing underground streams—the Minyé sinkhole (Papua New Guinea) and Cedar Sink (Mammoth Cave, Kentucky) are notable examples in which open depressions reveal passage of subterranean water. Hydrologically, sinkholes act as direct links between surface and groundwater systems: they route surface drainage into cave networks, store water in subsurface conduits, and modify local and regional groundwater flow paths, thereby influencing drainage patterns and river tributary behavior.
Because sinkhole occurrence is controlled primarily by subsurface geology and hydrology rather than surface topography alone, they may arise in low-lying basins that concentrate runoff or on upland, dry terrain where active percolation and soluble substrata coexist. This spatial variability has important consequences for land-use planning, infrastructure siting, and geohazard assessment.
On 2 July 2015 the European Space Agency’s Rosetta mission reported actively changing pits on the nucleus of comet 67P/Churyumov–Gerasimenko. Morphological analysis indicates these depressions are collapse features produced by mechanical failure of the comet’s surface and shallow subsurface strata, analogous in form and process to terrestrial sinkholes but driven by loss of subsurface volatiles rather than groundwater removal. The pits are spatially and temporally associated with localized, episodic jets of gas and dust, so they function both as collapse structures and as vents for volatile-driven outbursts; this association implies a two‑way coupling in which outgassing can induce structural failure and collapse can expose additional volatile reservoirs to further activity. The Rosetta observations thus furnish direct evidence that small icy bodies undergo ongoing geomorphic modification through subsurface volatile depletion, sinkhole formation and transient outbursts, with important implications for models of cometary evolution and the interaction between interior processes and surface morphology.
Artificial processes
Anthropogenic activities produce a distinct class of ground-surface collapses that are often—imprecisely—lumped together with natural karst sinkholes. Accurate classification requires identifying the initiating mechanism: some collapses originate from ruptured utility conduits or failing engineered structures, others from abandoned mine workings or purpose-built subsurface storage caverns. The term “sinkhole” is therefore insufficient unless the causal process (e.g., pipe failure, mining void roof collapse, salt-cavern instability, or chemical dissolution) is specified.
Urban infrastructure failure is a frequent cause of rapid, localized collapse. A well-documented example occurred in 2005 at a Georgia Tech parking lot, where rainwater migrated through pavement, entrained soil and flushed it into a ruptured sewer line, producing a roughly 32-foot (9.8 m) deep void. That case illustrates how surface-water infiltration combined with failed subsurface infrastructure can precipitate abrupt and deep loss of ground support.
Human-induced collapses also arise from subterranean voids created by resource extraction and engineered storage. Roof failures above abandoned mine workings are a common hazard, and collapses have been observed where salt caverns—often hosted in salt domes—are used for fluid or hydrocarbon storage. Documented examples in the United States include salt-dome storage complexes in Louisiana, Mississippi, and Texas.
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Hydrologic modifications and groundwater management substantially influence collapse risk. Large-scale fluid withdrawal (overpumping of groundwater or extraction of other subsurface fluids) reduces pore support and can provoke gradual subsidence or sudden collapse of cavity roofs. Similarly, altering surface drainage or introducing concentrated diversion channels can increase infiltration into previously stable or unsaturated zones, accelerating piping, erosion, or dissolution processes that undermine the land surface.
Finally, surface loading and land-use change interact with subsurface stability. The placement of heavy loads—industrial ponds, runoff storage basins, or other substantial surface weights—over preexisting voids can exceed the remaining roof strength of those cavities and trigger collapse. Assessing anthropogenic collapse hazard therefore requires integrating knowledge of subsurface voids, hydrologic change, infrastructure condition, and imposed surface loads.
Solution (dissolution) sinkholes form where percolating water chemically removes soluble bedrock, most commonly limestone, beneath an intact soil mantle. Dissolution proceeds preferentially along pre‑existing discontinuities—joints, fractures and bedding planes—so the growth of voids is controlled by the rock’s structural fabric; the shape, orientation and connectivity of the subterranean conduits therefore reflect these features. As cavities enlarge, the overlying soil progressively collapses or settles into the created porosity, producing the characteristic shallow surface depressions associated with dissolution sinkholes.
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This development can be understood as a three‑stage mass‑transfer and subsidence sequence: (1) infiltration and chemical weathering of carbonate rock, (2) progressive enlargement of pore space and conduits along structural planes, and (3) downward migration and compaction of soil into the cavities, culminating in surface subsidence. Essential conditions include a soluble lithology (e.g., limestone), a continuous, mobile soil cover, persistent water supply and pathways, and structural discontinuities to guide dissolution. Because these depressions are visible indicators of subsurface karstification and potential ground‑stability issues, geological surveys and agencies (e.g., USGS) map and monitor them as part of subsidence‑hazard assessment.
Cover‑subsidence sinkholes
Cover‑subsidence sinkholes form where unconsolidated surface materials progressively migrate into voids developed in underlying soluble bedrock, most commonly limestone. These subsurface openings arise through karst processes—chemical dissolution by circulating groundwater—that enlarge fractures and cavities into which overlying particles slowly settle. Because the infill occurs incrementally, the surface response is dominantly gradual subsidence rather than sudden collapse.
The mechanical behaviour of the cover largely controls the subsidence style. Thin, well‑sorted, granular soils with low cohesion and sufficient permeability transmit grains downward into cavities, producing slow, continuous lowering. In contrast, thick, cohesive, or heterogeneous covers can bridge openings for long periods and, if the bridge fails, produce abrupt collapse; thus cover properties (thickness, grain size, permeability, cohesion) determine whether subsidence or collapse predominates.
Morphologically, cover‑subsidence features tend to be broad, gently sloping, bowl‑shaped depressions that expand and deepen over time as successive sediment grains infill subsurface voids. Their incremental development generally presents a lower immediate risk of catastrophic failure than cover‑collapse sinkholes, but can result in substantial long‑term lowering of the land surface and progressive damage to built and agricultural environments.
From a planning and hazard perspective, cover‑subsidence sinkholes are characteristic of karst landscapes underlain by carbonate rock and are important for land‑use decisions, infrastructure design, and groundwater protection because they create direct pathways between surface and subsurface. Detection and monitoring combine routine field inspection for progressive depression, geotechnical probing, geophysical methods such as ground‑penetrating radar and electrical resistivity, and repeat topographic mapping or remote sensing time series. The U.S. Geological Survey recognizes cover‑subsidence as a distinct form of karst subsidence and provides classification schemes and guidance to support mapping and risk assessment.
Cover‑collapse sinkholes
Cover‑collapse sinkholes (commonly termed “dropouts”) form where loose, unconsolidated soils or sediments overlie soluble bedrock—typically limestone—and migrate downward into pre‑existing subterranean cavities. In karst terrains, progressive infilling of these voids by overlying material eventually outpaces the capacity of the surface layer to span or support the opening; once that support is lost, the surface fails suddenly rather than subsiding gradually.
Because the failure is localized and rapid, cover‑collapse events can cause severe, often unpredictable damage to roads, buildings, utilities and other infrastructure. Anthropogenic changes to hydrology in karst areas—such as redirecting runoff, altering infiltration, pumping groundwater or concentrating flow—can enhance soil removal into subsurface conduits and thereby trigger or accelerate collapses. The occurrence of cover‑collapse sinkholes is fundamentally controlled by the presence of dissolutional voids in soluble bedrock. Geological classifications used by agencies such as the U.S. Geological Survey distinguish cover‑collapse from cover‑subsidence sinkholes; the latter involve more gradual settling into voids, indicating a continuum of sinkhole behavior in karst landscapes rather than a single mode of failure.
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Pseudokarst sinkholes
Pseudokarst sinkholes are closed surface depressions that resemble karst dolines in form but arise from non-solutional processes. Their origin is mechanical, thermal, volcanic, cryogenic, or anthropogenic, or derives from the removal or decay of unconsolidated sediments rather than from chemical dissolution of soluble bedrock (carbonate or evaporite). The central distinction from true karst is therefore genetic: similar morphology can mask fundamentally different formative mechanisms.
A broad suite of processes produces pseudokarst. Volcanic terrains commonly generate collapses through the failure of lava tubes or voids within basaltic flows. In unconsolidated alluvium, subsurface piping and suffosion remove support and induce progressive collapse. Cryogenic settings create depressions when ground ice melts or permafrost thaws. Human activities—abandoned mines, quarries, buried infrastructure—can leave voids that subsequently collapse. Coastal undermining and the collapse of cavities formed by organic decay or sediment transport driven by groundwater also produce pseudokarst morphology. The specific process determines both the timing and style of collapse (gradual sagging versus sudden failure).
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Substrate properties and pre-existing void structures control where pseudokarst develops. Unlike solutional karst, which requires soluble carbonates or evaporites, pseudokarst occurs in volcanic rock, unconsolidated sands, silts and gravels, frozen ground, and anthropogenically altered strata. Mechanical strength, grain size, permeability and the presence of created or inherited cavities therefore govern susceptibility and geometry of depressions.
Morphologically, pseudokarst depressions range from small hollows to large collapse dolines and may be circular, irregular, steep-sided, or elongate, depending on mechanism and anisotropy in the substrate. Because planform and profile can closely mimic solutional sinkholes, field identification based on shape alone is unreliable; process-based evidence is required to distinguish origins.
Hydrologic behaviour also differs from true karst. Solutional sinkholes commonly connect to persistent conduit networks that control regional carbonate drainage; pseudokarst features more often have limited, transient or perched drainage, are infilled with unconsolidated sediment, or connect to anthropogenic voids. Consequently their impact on groundwater flow and contaminant transport is variable and less predictable than in established karst aquifers.
Discrimination between pseudokarst and karst requires integrated investigation: detailed geologic and surficial mapping, sediment and lithologic sampling, geophysical methods (e.g., ground‑penetrating radar, seismic refraction), and remote sensing to characterize morphology and subsurface structure, supplemented by historical and land‑use records to reveal anthropogenic causes. Hydrochemical analyses can further indicate whether carbonate dissolution is a driving process.
From an applied perspective, pseudokarst features present significant geohazard and engineering challenges—sudden collapse, structural damage, altered surface and subsurface drainage, and potential contamination pathways for shallow aquifers. Effective hazard assessment and mitigation must be tailored to the formative process (for example, monitoring thaw in permafrost landscapes versus securing voided mine workings), since strategies appropriate for solutional karst are not universally applicable.
Human‑accelerated sinkholes
Soil‑collapse sinkholes pose the greatest hazard in karst terrains because they result from downward migration of surficial soil into pre‑existing rock voids; when the process of soil infill is accelerated, failures that would otherwise take millennia can occur within years. Fluctuations in groundwater drive a characteristic two‑stage weakening: rising water through fissures reduces soil cohesion, and subsequent drawdown permits the weakened soil to drain into cavities where turbulent flow transports the material away, preventing re‑infilling and initiating progressive collapse.
Anthropogenic modification of surface and subsurface hydrology greatly increases collapse likelihood. Activities that concentrate runoff or alter recharge—such as clearing vegetation, ditching, installing pipelines, sewers, water and storm drains, drilling, and creating impervious surfaces (roads, roofs, parking lots)—focus surface water into discrete sink points and fissures and thereby raise local downward fluxes well above natural recharge. Quarries and other engineered dewatering can be especially hazardous; a dramatic example attributed to man‑made lowering of a quarry water level is the large 1972 collapse near Montevallo, Alabama (the “December Giant” or “Golly Hole”), which consumed over three acres and measured roughly 130 m × 105 m × 45 m.
Some induced collapses give precursory signs—surface cracking, sagging, misaligned doors, or audible cracking—but many occur with little or no warning, underscoring the need for comprehensive site investigation in karst regions. Because the spatial distribution of existing sinkholes often mirrors underlying cavities, areas of high sinkhole density are the most likely loci for further collapse and should be treated as high‑risk zones during planning and assessment.
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Most sinkhole disasters are preventable through targeted geological and hydrological characterization. Geotechnical practice in karst must go beyond standard bearing‑capacity and settlement analysis to include focused searches for cavities and rock discontinuities; the irregularity of the soil–rock interface typically requires far denser subsurface sampling (borings and cores) than non‑karst sites. Land‑use recommendations therefore emphasize avoiding or minimizing surface‑altering activities, preserving natural drainage, and reducing short‑ and long‑term groundwater variability by controlling recharge and preventing concentrated runoff. Professional bodies such as the American Society of Civil Engineers advocate integrating sinkhole‑collapse potential into land‑use planning and notifying the public where life may be at risk.
Economic losses from karst processes are substantial and probably underreported. The U.S. Geological Survey estimated direct repair costs in the United States at a minimum of about $300 million per year (averaged over the prior 15 years as of 2015), while recognizing data gaps. In the United States, the greatest sinkhole damage occurs in southeastern and central states—Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania—reflecting both extensive soluble rock and intensive land use. Internationally, significant karst hazard regions include the Ebro Basin (Spain), Sardinia and the Italian peninsula, southern England’s Chalk, Sichuan province (China), Jamaica, France, Croatia, Bosnia and Herzegovina, Slovenia, and large portions of Russia, indicating widespread global exposure to sinkhole and subsidence risk.
Occurrence
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Sinkholes are most common in karst terrains underlain by soluble carbonate bedrock, where dissolution creates a landscape pitted with numerous collapse and subsidence features. In such settings surface runoff is frequently captured by depressions, allowing water to drain directly into the subsurface; this routing yields extensive subterranean drainage networks, rapid aquifer recharge, and direct surface‑to‑groundwater connections exemplified by the Alapaha River near Jennings, Florida, which loses its entire surface flow into a sinkhole feeding the Floridan Aquifer.
The propensity for sinkhole development depends on lithology, stratigraphy and hydrology. Thick, relatively homogeneous limestones subject to high groundwater throughput—often driven by heavy rainfall—are particularly susceptible because enhanced flow accelerates chemical weathering and void enlargement; these conditions account for exceptionally large collapses such as those in the Nakanaï Mountains (New Britain). Likewise, where permeable carbonate layers overlie insoluble or less permeable rocks, concentrated underground rivers can erode extensive cavities at the lithologic contact, generating some of the world’s largest vertical collapse features.
Globally documented extremes illustrate these controls and their variations. The Xiaozhai Tiankeng in China attains a depth of 662 m, and the giant sótanos of central Mexico display profound vertical development in carbonate settings. Sistema Zacatón (Tamaulipas, Mexico), a cluster of more than twenty karst openings developed by volcanically heated, acidic groundwater, produced the deepest water‑filled sinkhole (Zacatón) and locally precipitated travertine that has sealed upper apertures. Not all large collapses occur in carbonates: Sima Humboldt and Sima Martel in Venezuela demonstrate that competent siliciclastic rocks such as sandstone can also host deep collapse shafts when geological and erosional conditions favor subsidence.
Regional and anthropogenic factors further modulate occurrence. In Florida, sinkhole frequency correlates with carbonate age and thickness: central Florida’s 15–25 Ma limestones are highly susceptible, whereas the much younger (~120 ka) marginal deposits show few collapses. The Murge plateau in southern Italy provides a contrasting example of extensive karstification on carbonate plateaus. Human activities that concentrate water on soluble substrates—such as engineered retention basins—can locally trigger sinkhole formation. Non‑classic analogues occur where other processes produce rapid removal or pressure change in the subsurface; for example, methane release from the Arctic seabed has induced large seafloor collapse features comparable, in process if not in setting, to terrestrial sinkholes.
Historical awareness of large karst cavities is longstanding: features such as France’s Gouffre de Padirac have been recorded since antiquity and explored in the modern era, reflecting the persistent prominence of these phenomena in carbonate landscapes worldwide.
Human uses
Sinkholes have long been appropriated by humans as convenient receptacles for refuse, a practice that can introduce surface contaminants directly into subsurface voids. In karst and other permeable terrains many sinkholes communicate with aquifers, allowing disposed material to circumvent soil‑level filtration and enter groundwater quickly; this rapid transfer elevates the risk of widespread aquifer contamination and consequent public‑health problems for reliant communities.
Beyond disposal, certain sinkhole types occupy important hydrological and cultural roles. In the Yucatán Peninsula, for example, collapse dolines known as cenotes provide direct access to the regional groundwater network and historically served as loci for ritual deposition by the ancient Maya, linking a geomorphological form to both practical water use and archaeological significance.
From an exploratory perspective, deep vertical sinkholes and those integrating into cave systems create demanding technical environments. Vertical caving into these features requires specialized rigging and skill, while water‑filled shafts necessitate open‑water or cave‑diving expertise and equipment, markedly increasing operational hazards even for experienced speleologists and divers.
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Geographically, sinkholes and related features occur in diverse settings and attract scientific, recreational, and cultural interest. Notable instances include the Zacatón cenote in Mexico (recognized for exceptional water depth), Boesmansgat in South Africa, the Sarisariñama sinkhole complexes on a Venezuelan tepuy, Mexico’s Sótano del Barro, and the multiple collapse features around Mount Gambier, Australia. Where carbonate reefs or islands have collapsed to form deep marine shafts, the resulting blue holes present steep, often near‑vertical conduits to the sea that are prized by divers for their depth, water clarity, and submerged structure.
Collectively, these uses and characteristics illustrate sinkholes’ multifaceted geographic importance: as potential conduits of contamination with attendant health implications; as reservoirs of cultural and archaeological material; as technically challenging natural systems for scientific investigation and recreation; and as striking geomorphological phenomena distributed across a range of geological settings.
Local names
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Large, visually striking sinkholes in carbonate and karst terrains occur worldwide and have long been recognized by local populations; contemporary classification therefore blends vernacular toponyms with scientific terminology. The Great Blue Hole off Ambergris Caye, Belize, exemplifies an iconic, water‑filled karst feature whose common name reflects both its appearance and its cultural recognition.
Regional terms capture specific morphologies and origins. In southern France the Occitan word aven (literally “pit cave”) denotes steep, pit‑like karst sinks and has been adopted in karst literature. The Bahamas host so‑called “black holes,” round, water‑filled pits formed by solution of carbonate mud; their dark aspect is caused by a dense, phototropic microbial layer concentrated at middepth (c. 15–20 m), which strongly absorbs light and alters local thermal conditions. “Blue hole” originated for the deep submerged pits of the Bahamian platform but is now widely used for any deep, clear‑water sink in carbonate rock; the intense blue color arises from selective transmission and backscatter of short‑wavelength light in deep, transparent water.
Mesoamerican terminology includes cenote, the characteristic water‑filled sinkholes of the Yucatán Peninsula and Belize that are closely linked to the region’s submerged karst aquifer; clusters of cenotes above the rim of the Chicxulub impact were instrumental in recognizing that buried crater. In Slovenia the term dolina (literally “valley” or “dale”) is applied internationally to closed karst depressions, reflecting their valley‑like form. Along the Karst Plateau and adjacent frontier areas of Friuli‑Venezia Giulia, Croatia and Slovenia the Friulan Italian foiba (from Latin fŏvea, “pit”) names local sinkholes and chasms.
Several countries use distinct names for very large or vertical features: Mexican sótanos denotes giant vertical pits formed by collapse or solution; Chinese tiankengs (“sky holes”) designate exceptionally large sinkholes—commonly >250 m in depth and diameter—with steep walls and collapse origin, many examples being located in China. In New Zealand karst the term tomo describes collapse and solution depressions used in regional geomorphological accounts. Croatian karst employs multiple local terms—vrtača, ponikva, dolac and dô—to distinguish regional varieties of closed depressions and karstic pits.
These diverse toponyms encode regional language, morphology and genesis, and they remain important for describing, classifying and interpreting karst sinkholes in both local and scientific contexts.
Piping pseudokarst (Guatemala City, May 2010)
In May 2010 a sudden, large vertical collapse opened in Guatemala City during torrential rains associated with Tropical Storm Agatha. The aperture measured on the order of 20 m in diameter and about 30 m deep and engulfed a three‑story building and an adjacent house. A comparable collapse had occurred nearby in February 2007, indicating that the event was part of a recurring urban geohazard rather than an isolated anomaly.
The feature is best interpreted as a piping pseudokarst rather than a solution sinkhole: it did not form by chemical dissolution of carbonate rock but by mechanical removal of unconsolidated material. The collapses developed within weak Quaternary volcanic deposits that, although friable, can temporarily support near‑vertical voids. Flowing water—principally stormwater infiltration combined with leaking water mains—washed fine volcanic grains from these deposits and progressively mobilized coarser particles, producing subsurface pipes and enlarging cavities (soil piping).
When those underground voids attained a critical size the overlying roofs failed catastrophically, producing abrupt vertical collapses at the surface. The Guatemala City case exemplifies how intense precipitation, deficient urban drainage and leaking infrastructure interacting with susceptible volcanic substrata can generate sudden ground failure in urban areas, posing persistent risks to buildings and infrastructure.
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Crown hole
A crown hole is a form of surface subsidence produced when human-made underground voids—such as mine workings, quarries or military excavations—remove or undermine the material that supports overlying ground, leading to destabilization and eventual sagging or collapse of the surface. Mechanically, crown holes result from loss of support, weakened strata and altered stress regimes above excavations; these conditions allow progressive settlement or sudden failure of the ground above the void.
Anthropogenic activities that create the conditions for crown-hole formation include both extractive industries (active and abandoned mines, quarrying) and engineered trenches associated with military operations. Each produces void space or disrupted ground fabric that increases the probability of surface collapse, with the risk persisting after operations have ceased when backfill is ineffective or roof-supporting pillars deteriorate.
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European examples illustrate the phenomenon across different geological and historical contexts: subsidence above World War I trench systems at Ypres (Belgium) shows how historic military networks can later induce crown holes; occurrences near mine workings around Nitra (Slovakia) document the link with contemporary or former mining; a crown hole above a limestone quarry in Dudley (England) highlights the susceptibility of quarried carbonate bedrock; and the collapse above the abandoned gypsum mine at Magheracloone (Ireland) demonstrates that evaporite workings can produce hazardous subsidence long after extraction ends. These cases underline that crown holes can arise in diverse lithologies and temporal settings, and that anthropogenic voids constitute a distinct class of sinkhole hazard.
Notable example: Bimmah (the Falling Star Sinkhole), Oman
Bimmah, commonly called the Falling Star Sinkhole, is a well‑known sinkhole in Oman that exemplifies the larger class of collapse depressions formed in soluble rock. Such features range from modest hollows to some of the most dramatic collapsed landforms on Earth and are therefore routinely included in regional and global inventories of major sinkholes.
Geomorphologically, sinkholes like Bimmah develop where subsurface voids form faster than the overburden can be supported, typically through chemical dissolution of soluble bedrock (for example limestone, dolomite or gypsum) and the eventual mechanical failure of cave roofs or saturated sediments. The result is a steep‑sided depression at the surface that records the interplay of rock solubility, groundwater flow and structural weaknesses.
Hydrologically, sinkholes provide direct surface–groundwater connections: they can capture runoff, convey water into aquifers, serve as points of spring discharge, and—in coastal settings—facilitate mixing between fresh and marine waters with attendant effects on local salinity and groundwater direction. These roles make sinkholes important elements in karst hydrogeology and aquifer recharge dynamics.
From an environmental and societal perspective, large sinkholes create distinct ecological niches and attract tourism but also pose significant hazards to infrastructure and land use through sudden collapse, subsidence and altered groundwater pathways. Consequently, documenting features such as Bimmah within systematic inventories, combined with detailed mapping, subsurface investigation (e.g., geophysics and boreholes) and ongoing monitoring, is essential for understanding karst evolution, assessing hazards, informing conservation, and guiding risk‑reduction measures.
Sinkholes in Africa
African sinkholes display a range of karst morphologies and hydrological settings, from deep inland submerged shafts to steep-sided collapse lakes and marine blue holes. In South Africa, Boesmansgat is a freshwater shaft that penetrates approximately 290 m (950 ft) vertically, making it one of the deepest known inland submerged karst features and a prominent expression of regional limestone dissolution. In Zambia, Lake Kashiba occupies about 3.5 hectares (8.6 acres) and reaches roughly 100 m (330 ft) of water-filled depth; its steep walls and limited surface extent reflect localized collapse and subsidence driven by subsurface dissolution. Off the Egyptian coast at Dahab, the Blue Hole is a circular marine sinkhole descending to about 130 m (430 ft) and containing a submerged archway that links to the Red Sea at near 60 m (200 ft); this structural configuration creates both unique ecological connectivity and pronounced occupational hazards for freedivers and scuba divers, with the deeper passages associated with recurrent fatal incidents. Collectively, these examples illustrate the diversity of karst sinkhole forms in Africa and underscore their geological significance and safety implications.
Asia
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Sinkholes across Asia exhibit a wide range of morphologies—from shallow coastal depressions and vertical pit caves to enormous tiankengs and subaqueous chasms—reflecting diverse karst processes and settings. In southern Turkey the Akhayat sinkhole is a prominent surface collapse roughly 150 m across and approaching 70 m in depth, a conspicuous feature in the regional landscape. On the Arabian Peninsula the Well of Barhout in Yemen is notable for its deep, near-vertical shaft that reaches about 112 m, while Oman hosts both the shallow, culturally named Bimmah sinkhole (roughly 30 m deep) and the vast Teiq sinkhole, which contains an estimated 9×10^7 m3 of void space and plunges to some 250 m; several perennial wadis feed waterfalls into the latter, linking surface drainage and subsurface voids.
In the eastern Mediterranean, Lebanon’s Baatara Gorge couples a dramatic sinkhole with an associated waterfall within karstified limestone near Tannourine. China provides some of the most extreme examples of tiankeng development: the Dashiwei tiankeng in Guangxi descends over 600 m and supports an isolated forest patch at its base, illustrating depth-driven ecological isolation; a major cluster in southern Shaanxi’s Daba Mountains spans thousands of square kilometers, with the largest individual depression about 520 m across and some 320 m deep; and the Xiaozhai tiankeng in Chongqing is a double-nested structure attaining a maximum depth of about 662 m, among the deepest and most complex known. Offshore, the Dragon Hole south of the Paracel Islands represents the deepest documented marine sinkhole, at roughly 301 m, emphasizing that sinkhole formation can extend into subaqueous environments.
Collectively these features underscore the importance of karst processes in shaping Asian terrains, their role in channeling surface and groundwater (often producing waterfalls and springs), and their capacity to create isolated habitats and large subsurface voids with significant geomorphological and ecological implications.
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Dean’s Blue Hole, off the coast of Long Island in the Bahamas, is a marine karst sinkhole reaching a vertical depth of 203 m (666 ft), making it one of the deepest known submerged shafts relative to surrounding coastal seafloor. Classified as a “blue hole,” it formed by dissolution of near‑surface carbonate bedrock during Pleistocene lowstands; subsequent post‑glacial sea‑level rise flooded the void, leaving a near‑vertical shaft that connects the sea surface to much deeper subsurface cavities and the adjacent submarine environment.
The feature’s narrow surface aperture and abrupt drop create a striking underwater morphology: a rapid transition from shallow coastal waters to great depth produces persistent vertical gradients in temperature and salinity (thermoclines and haloclines) and sharp light attenuation with depth. These physical stratifications influence both ecological zonation and human use, shaping accessible pathways for divers while imposing physiological and safety constraints.
Oceanographically and ecologically, deep marine sinkholes like Dean’s Blue Hole often show reduced deep‑water circulation, distinct layered water masses, and attendant biogeochemical gradients. Such conditions support specialized biological assemblages adapted to low‑light, low‑exchange environments and preserve sedimentary records that are valuable for paleoenvironmental study. At the same time, the confined nature of these systems makes them susceptible to disturbance from pollution, overuse, or changes in land‑sea connectivity.
Dean’s Blue Hole also exemplifies the intersection of physical geography and human activity: its extreme but readily reachable depth, sheltered surface conditions, and straight vertical profile have made it an international center for free‑diving competitions and recreational diving, contributing to local tourism and the island economy. These uses highlight the need for site‑specific safety protocols and conservation measures that balance recreational and scientific values while recognizing the broader role of carbonate island karst in shaping the Bahamas’ coastal landscape.
Central America — Sinkholes
The Great Blue Hole off Belize is a near-circular, submerged karst collapse feature approximately 124 m deep. Its morphology—an extensive, cavernous sinkhole beneath the sea—records a history of limestone dissolution and roof failure that predates its present marine setting.
Preserved speleothems recovered from depth, many now tilted from their original orientations, demonstrate that cave formation occurred in subaerial, air‑filled conditions. The orientation and preservation of these stalactites provide direct field evidence of former cave ceilings and bed orientations, and their reorientation by collapse or deformation constrains the sequence of local landscape evolution and relative sea‑level rise that produced the present inundated state.
By contrast, the dramatic sinkholes that opened in Guatemala City in 2007 and 2010 are examples of episodic urban collapse rather than deep karstic cavities. These features developed rapidly in a built environment and are commonly driven by near‑surface processes such as subsurface erosion and piping, failed drainage or utility conduits, unstable volcanic deposits or unconsolidated soils, and intense rainfall; their impacts are acute for infrastructure and land use.
Together these cases illustrate divergent geomorphic origins and hazard profiles: the Great Blue Hole typifies a large, speleothem‑bearing coastal karst collapse linked to long‑term dissolution and sea‑level change, whereas urban sinkholes in Guatemala arise from localized, anthropogenically influenced subsurface degradation. Consequently, assessment, monitoring, and mitigation strategies must be tailored to the distinct mechanisms, spatial scales, and risk contexts of natural coastal karst versus urban collapse phenomena.
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Europe — Sinkholes
Sinkholes across Europe exemplify the range of karst phenomena from shallow collapse depressions to exceptionally deep, water-filled shafts, reflecting variations in lithology, structural control and groundwater dynamics. These features are important both for understanding regional hydrogeology and for assessing geohazard and land-use implications.
Some of the continent’s most extreme examples are deep phreatic shafts with extensive flooded sections. The Hranice Abyss (Moravia, Czech Republic) is the internationally recognised deepest known submerged cave, with a confirmed exploration depth of 473 m (404 m below the water surface) recorded in 2016, illustrating pronounced vertical karstification and significant speleogenetic potential. Near Rome, Pozzo del Merro is a wide conical pit (top diameter ~80 m) that descends into a flooded shaft approaching 400 m, similarly indicating deep phreatic conditions. Croatia’s Red Lake (Crveno jezero) demonstrates dramatic collapse morphology: nearly vertical walls attain an overall pit depth of about 530 m, and a perennial lake fills a water column of roughly 280–290 m, producing a deep submerged lacustrine karst environment.
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Other sites illustrate the diversity of human interactions and risks associated with sinkholes. France’s Gouffre de Padirac (Lot) features a 103 m vertical shaft (surface diameter ~33 m) leading to an underground lake and a mapped subterranean river system of about 55 km; its developed visitor access (descent of ~75 m and an organized boat tour) highlights the tourism and scientific value of accessible karst conduits. Conversely, the Vouliagmeni sinkhole in Greece—locally called the “Devil Well”—has a maximum vertical depth of ~35.2 m and extends horizontally some 150 m; its confined submerged passages have proved lethal to divers (four reported fatalities), underscoring safety hazards inherent in complex subaqueous karst.
Smaller, yet geomorphologically distinct, collapse features also occur. Malta’s Maqluba near Qrendi is a circular karst collapse about 50 m across, roughly 15 m deep and covering ca. 4,765 m², typifying compact collapse morphology within carbonate terrains. In County Kerry, Ireland, the Pouldergaderry sinkhole (approx. 52.132639°N, −9.745944°W) is an ~80 m diameter, ~30 m deep depression that supports mature trees on its floor, occupies about 1.3 acres and appears on Ordnance Survey maps since 1829, indicating long-term landscape stability and historical recognition of karst landforms. Together, these European examples demonstrate the structural, hydrological and societal significance of sinkholes across a range of scales.
Mexico hosts a wide range of karst collapse features that illustrate the diversity of sinkhole morphology, genesis, and significance. These include exceptionally deep vertical shafts, actively enlarging collapses, biologically and culturally significant pits, and groundwater‑filled depressions whose vertical extent is governed by the water column.
The Cave of Swallows (Sótano de las Golondrinas) in San Luis Potosí is an extreme example of a collapsed karst shaft: a near‑circular, free‑fall vertical sinkhole with strongly overhanging walls and a measured rim‑to‑floor depth of about 372 m (1,220 ft), representing one of the largest uninterrupted vertical drops formed by roof collapse of a subsurface void. By contrast, the 2021‑documented Puebla sinkhole at Santa María Zacatepec is a rapidly developing collapse feature roughly 120 m (400 ft) across and initially about 15 m (50 ft) deep; its continued growth at the time of reporting highlights ongoing subsurface dissolution and instability of surficial soils and bedrock, with attendant geohazard implications for the locality. Sima de las Cotorras in Chiapas, a broad pit roughly 160 m (520 ft) across and 140 m (460 ft) deep, functions as a sheltered amphitheatre that sustains thousands of green parakeets and preserves ancient rock paintings on its inner walls, exemplifying how sinkholes can create distinctive microhabitats and serve as long‑used cultural sites. Zacatón in Tamaulipas, by contrast, is the world’s deepest known water‑filled sinkhole (approximately 339 m / 1,112 ft); as a flooded karst shaft, its depth is expressed through the water column and emphasizes the close coupling between karst collapse and regional groundwater systems.
Together these sites demonstrate sinkholes’ geomorphic extremes and their multifaceted roles in landscape evolution, hydrogeology, ecological refuge, cultural history, and local geohazard exposure.
United States — notable sinkholes
Sinkhole occurrences across the United States display a wide range of morphologies, sizes, depths and causal mechanisms, from broad enclosed karst basins to abrupt collapse pits and submarine blue holes. In the Cumberland Plateau of Tennessee, Grassy Cove exemplifies a large karst depression: an enclosed basin of about 13.6 km2 with a maximum depth near 42.7 m, illustrating landscape-scale solution and subsidence in carbonate bedrock. Sudden, catastrophic collapse is exemplified by the Golly Hole (the “December Giant”) in Calera, Alabama, which opened in a populated area on 2 December 1972 and measured roughly 91 × 99 m in plan with a vertical extent of about 35 m, demonstrating rapid subsidence hazards where near-surface cavities fail.
Florida provides multiple expressions of solution collapse and spring-fed sinkholes: nearshore submarine karst is represented by Amberjack Hole (≈48 km offshore of Sarasota) and Green Banana Hole (≈80 km offshore), both deep marine blue holes in the Gulf of Mexico. On the peninsula itself, near-circular solution lakes such as Kingsley Lake (≈8.1 km2 area, max depth ≈27 m) and collapse features like the Winter Park sinkhole (appearing 8 May 1981; diameter ≈110 m, depth ≈25 m, later filled to form Lake Rose) and the steep-walled Devil’s Millhopper in Gainesville (≈35 m deep, ~150 m across, with a spring-fed pond at the base) illustrate typical Floridian karst behavior where dissolution, groundwater flow and collapse interact.
Coastal-plain and anthropogenic examples in Louisiana and Texas demonstrate much larger and sometimes atypical consequences of subsurface voids. The Bayou Corne sinkhole in Assumption Parish affected roughly 25 acres and has been reported to extend to very large depths (reports near 230 m), reflecting extensive subsurface collapse in a low-relief coastal setting. Lake Peigneur near New Iberia underwent a dramatic human-triggered transformation when a salt‐mine collapse reconfigured a shallow lake (≈3.4 m original depth) into a deepened basin reported to reach on the order of 400 m—an extreme case of mine–surface water interaction. In Daisetta, Texas, recurrent sinkhole activity (most recently in 2008) has produced features up to ~190 m in diameter and ~45 m deep, indicating persistent subsidence susceptibility where subsurface cavities, extraction or fluid migration are active.
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Other distinctive sinkholes include the Blue Hole at Santa Rosa, New Mexico, a shaft-like artesian spring with a small surface aperture (~24 m) that widens to about 40 m at depth and is used recreationally for diving, and a gypsum-derived collapse within Capitol Reef National Park, Utah, where dissolution of evaporites produced a narrow, deep shaft roughly 15 m across and ~60 m deep. Collectively, these examples underscore the diversity of sinkhole genesis in the United States—controlled by lithology (carbonates versus evaporites), groundwater dynamics, marine settings, and human activities—and the corresponding variation in scale and hazard.
Harwoods Hole (Abel Tasman National Park, New Zealand)
Harwoods Hole is a prominent vertical shaft within Abel Tasman National Park, New Zealand, characterized by a perpendicular rim-to-floor depth of 183 m (600 ft). This measurement defines it as a substantial geomorphological void whose extreme verticality produces a distinct point of relief in the local landscape and markedly alters the connection between surface and subsurface environments.
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The shaft’s form implies concentrated vertical gradients in microclimate and presents significant fall and access hazards; these conditions make it an important site for vertical cave exploration (speleology) but one that demands specialized descent, mapping techniques, and rigorous safety management. As an integral component of the park’s suite of geological features, Harwoods Hole contributes to regional geomorphological diversity and informs land‑management decisions regarding visitor access, hazard mitigation, and conservation planning.
Sima Humboldt (Bolívar state, Venezuela)
Sima Humboldt is a single, steep-walled sinkhole incised into sandstone, with a rim-to-floor depth of 314 m (1,030 ft). Unlike typical solutional caves in carbonate rock, its sandstone lithology and high verticality create a pronounced, enclosed habitat on the sinkhole floor, where a distinct forested assemblage demonstrates pronounced ecological segregation and a self-contained microenvironment.
Western Cerro Duida (Cerro Duida, Venezuela)
The western flank of Cerro Duida comprises a composite geomorphological system of canyons and multiple collapse depressions. The deepest individual shaft within this network reaches 450 m (1,480 ft) when measured from an internal low rim, while the combined canyon–sinkhole complex attains about 950 m (3,120 ft) of total vertical relief, indicating a multi-tiered erosional and collapse structure rather than a solitary shaft.
Comparative interpretation and implications
Together these examples illustrate contrasting modes of deep-sinkhole development in northern South America: Sima Humboldt typifies an isolated, high-walled sandstone collapse with a strongly bounded biotic pocket, whereas the western Cerro Duida area represents an interconnected, large-magnitude canyon and collapse system with greater overall relief but more distributed internal shafts. These differences imply divergent formative histories, varying geomorphic complexity, and distinct potentials for microclimate formation and ecological isolation across Venezuelan karst-like features.