A doline is a closed depression in the ground formed by the dissolution of soluble bedrock, most commonly limestone, gypsum, or salt. The word comes from Slavic languages and is the preferred scientific term for what much of the English-speaking world calls a sinkhole. Dolines range from gentle, barely noticeable saucer-shaped dips a few meters across to dramatic collapse pits tens of meters deep, and they are the single most characteristic landform of karst landscapes worldwide.
How Dolines Form
The basic process is straightforward: slightly acidic water dissolves soluble rock underground, creating voids. Rainwater picks up carbon dioxide from the atmosphere and from soil, turning it into a weak carbonic acid. As this water seeps through cracks and bedding planes in limestone or evaporite rock, it slowly eats away the mineral structure. Over time, the voids enlarge into cavities. When the roof of a cavity can no longer support the weight above it, the surface sags or collapses, producing a doline.
The details vary depending on the rock type, the overlying soil, and the way water moves through the system. In areas underlain by thick beds of gypsum or halite (rock salt), dissolution can be far faster than in limestone because these minerals are much more soluble. In the Ebro River valley in northeastern Spain, for example, researchers mapped three distinct doline types in a single 50-square-kilometer stretch: large collapse sinkholes up to 50 meters across and 6 meters deep, broad shallow subsidence depressions reaching 850 meters in length, and hundreds of smaller cover-collapse sinkholes along the valley margin.1Earth Surface Processes and Landforms. The origin, typology, spatial distribution and detrimental effects of the sinkholes developed in the alluvial evaporite karst of the Ebro River valley downstream of Zaragoza city (NE Spain) Each formed by a different combination of rising groundwater dissolving evaporite beds and the overlying material responding to the loss of support beneath it.
The Main Types
Geologists classify dolines by how the surface material fails, not just by how the bedrock dissolves. The distinctions matter because each type behaves differently and poses different risks.
- Solution dolines: The bedrock itself is exposed or nearly so, and the depression forms by direct chemical dissolution at the surface. These tend to develop slowly, over centuries or millennia, and are often wide, shallow, and gentle-sided.
- Cover-subsidence dolines: A layer of soil or loose sediment sits on top of soluble rock. As the rock dissolves beneath, the cover gradually settles downward. The result is a slow, progressive sagging that can go unnoticed for years.
- Cover-collapse dolines: The most dramatic and dangerous variety. A soil arch forms over an expanding void in the bedrock. The arch holds for a while, sometimes supporting roads, buildings, or fields. Then it fails suddenly, often without warning. The mechanics involve either crack propagation through the soil walls surrounding the void or sloughing driven by pore water pressure that effectively pulls the walls apart.2Engineering Geology. Mechanics of upward propagation of cover-collapse sinkholes
- Suffosion dolines: Loose granular material like sand or gravel is washed downward into bedrock fissures by flowing water, causing the ground surface to subside. These tend to be smaller but can form relatively quickly.
Collapse sinkholes get most of the media attention because they can swallow cars, sections of road, and occasionally entire buildings in a matter of hours. But the slow-forming varieties are far more common and collectively affect larger areas of land.
When Humans Trigger Dolines
Natural dolines take centuries to develop. Human activity can compress that timeline dramatically. The most common trigger is pumping groundwater faster than it is replenished. When the water table drops, underground cavities that were previously filled with or partially supported by water lose that buoyancy. The effective weight on the cavity roof increases, and collapses that might not have happened for another thousand years happen now.
This pattern has played out in arid regions that rely heavily on groundwater for irrigation. In Saudi Arabia, a recent spate of sinkholes has been tied to the expansion of irrigated agriculture. Researchers found a clear spatial and temporal correlation between groundwater drawdown and new sinkhole formation, and confirmed at one specific site using subsurface imaging that the roof of an older, pre-existing cavity had been destabilized by the loss of water support.3Journal of Arid Environments. Sinkholes induced by uncontrolled groundwater withdrawal for agriculture in arid Saudi Arabia In western Iran, a similar story has unfolded over the past three decades: increased groundwater extraction for agriculture and industry, compounded by successive droughts, has lowered water levels enough to cause both widespread land subsidence and a jump in sinkhole occurrences.4Quarterly Journal of Engineering Geology and Hydrogeology. The effect of groundwater overexploitation on land subsidence and sinkhole occurrences, western Iran
Other human triggers include leaking water mains and sewers (which accelerate dissolution and erode soil into voids), vibration from construction or heavy traffic, and altering surface drainage so that water is concentrated onto vulnerable ground. Mining can also remove support from below. In regions with active karst, any significant change in how water enters, leaves, or sits in the subsurface can tip a precarious cavity toward failure.
Finding Dolines Before They Find You
One of the most frustrating things about cover-collapse dolines is that the void grows silently underground while the surface looks perfectly normal. By the time cracks appear in a building or a hole opens in a road, the subsurface damage is already extensive. This has driven a lot of interest in geophysical detection methods that can image the subsurface without digging.
Ground-penetrating radar (GPR) sends electromagnetic pulses into the ground and reads the reflections to build a picture of what lies below. It works well for finding voids and disturbed zones in the first several meters of depth, especially in dry, sandy, or gravelly soils. Electrical resistivity tomography (ERT) passes current through the ground and maps how easily different materials conduct it; air-filled cavities and water-saturated clay show up as stark contrasts against solid rock. Used together, GPR and ERT can image small to medium cavities roughly one to four meters across, at depths reaching about eight meters.5Engineering Geology. Assessing the risk of subsidence of a sinkhole collapse using ground penetrating radar and electrical resistivity tomography Researchers in Iran have also found that combining GPR surveys with analysis of soil properties helps identify the zones most prone to collapse.6Geoderma Regional. Assessment of zones prone to sinkhole using ground penetrating radar and soil properties in Central Iran
These methods are practical for targeted surveys of building sites, road corridors, and infrastructure corridors, but scanning an entire karst landscape at high resolution remains expensive and time-consuming. For broader coverage, remote sensing from satellites and aircraft can identify clusters of existing dolines that hint at where new ones are likely to form, and LiDAR surveys can detect subtle surface depressions invisible to the naked eye.
The Cost of Ignoring Them
When a doline opens beneath infrastructure, the bill arrives fast. A risk assessment for a road built over sinkhole-prone ground in Europe estimated that over a 50-year period, about ten cavities between two and four meters wide were expected to form beneath the embankment, with average economic losses of roughly 700,000 euros per sinkhole and total unmitigated losses reaching seven million euros.7Elsevier (Engineering Geology). Application of risk, cost–benefit and acceptability analyses to identify the most appropriate geosynthetic solution to mitigate sinkhole damage on roads That is a single stretch of road. Scale the problem to entire regions underlain by karst, and the cumulative costs to roads, railways, pipelines, and buildings are staggering.
Mitigation options range from simple to elaborate. Geosynthetic reinforcement layers beneath road surfaces can bridge small voids and buy time for detection and repair before a full collapse occurs. Grouting involves injecting cement or chemical mixite into the ground to fill voids and stabilize loose soil. In some cases, engineers divert surface water away from vulnerable areas or install monitoring systems that detect ground movement in real time. None of these approaches eliminate the risk entirely in active karst, but they can reduce the frequency and severity of sudden failures substantially.
Dolines as Ecological Refuges
Not everything about dolines is hazardous. Ecologists have discovered that dolines create microclimates that shelter species unable to survive in the surrounding landscape. The bowl-shaped topography traps cool, moist air at the bottom while deflecting wind and moderating temperature swings. In forested karst landscapes, doline bottoms act as “cool islands,” capturing and storing substantially more water and nutrients in their soil than the surrounding plateau. Research has shown that these microclimatic and resource differences are significantly linked to the vegetation patterns found there, with many vascular plant species restricted almost entirely to doline microsites.8iScience. Microclimate buffering and unique ecological biodiversity refugia within dolines
This makes dolines function as microrefugia, places where species can persist through periods of unfavorable climate. Studies have confirmed that dolines support unique species assemblages and cooler, moister conditions consistent with their role as biodiversity hotspots.9Ecological Indicators. Disturbance and topography shape the capacity of microrefugia to support unique biodiversity In a warming world, these natural depressions could become increasingly important for conserving cold-adapted species that are being squeezed out of their historical ranges. Conservation planning in karst regions is starting to account for this, mapping doline networks not as hazards to build around but as habitats worth protecting.
Groundwater Pollution and Karst Vulnerability
Dolines are direct conduits to underground water systems. Any pollutant that enters a doline, whether from agricultural runoff, a leaking septic system, or illegal dumping, has a fast track to the aquifer below. Unlike in non-karst landscapes where soil layers filter contaminants gradually, water entering a doline can reach the water table in hours or days with minimal filtration.
Research tracking solute transport from sinkholes to karst springs has quantified the problem. During heavy rainfall, nutrients from agricultural land dissolve into overland flow and pour into dolines, causing nitrogen and phosphorus concentrations to spike. At one monitored karst system, total inorganic nitrogen reached over 11 milligrams per liter and phosphate exceeded 1 milligram per liter after storms. Pollutant concentrations at the karst spring outlet were two to three times higher than natural background values after rainstorms, even though some storage and dilution occurred along the flow path.10Journal of Hydrology. Accumulation, conversion and storage of solute from sinkholes to karst spring under concentrated recharge conditions The accumulation and long-term storage of pollutants in karst aquifers represents a slow-building contamination problem that is difficult and expensive to remediate once established.
For the roughly quarter of the world’s population that depends on karst groundwater for drinking water, this is a serious concern. Land-use planning around dolines matters enormously: setback distances for agriculture, restrictions on waste disposal, and stormwater management all reduce the risk of contaminating the aquifer through these natural funnels.
Archives of Ancient Climate
Because dolines collect and trap sediment over long periods, they serve as natural archives of environmental history. Pollen grains, organic matter, and mineral particles settle into doline lakes and slowly build up layered records that scientists can drill and read like a book.
One of the most celebrated examples is Lake Tulane in south-central Florida, a sinkhole lake whose sediment core extends back roughly 50,000 years. The 18.5-meter core, recovered from beneath 22.7 meters of water and dated using twenty radiocarbon measurements, revealed alternating phases of pine-dominated and oak-ragweed-dominated vegetation. The pine peaks correlated in detail with Heinrich events, episodes of massive iceberg discharge into the North Atlantic that disrupted ocean circulation. This demonstrated a direct link between continental vegetation shifts in Florida and oceanic climate events thousands of kilometers away.11Palaeogeography, Palaeoclimatology, Palaeoecology. Pre-holocene and holocene pollen records of vegetation history from the Florida peninsula and their climatic implications
Similar work has been done in the Caribbean. Sediments from Blackwood Sinkhole on Abaco Island in the Bahamas have been analyzed using plant wax hydrogen and carbon isotopes, sterol biomarkers, and pollen to reconstruct precipitation and vegetation changes over the past 3,000 years.12Organic Geochemistry. Plant wax evidence for precipitation and vegetation change from a coastal sinkhole lake in the Bahamas spanning the last 3000 years In the Mediterranean, dolines on Crete have been found to contain loose fill up to 20 meters deep, offering rich records of environmental history in a region where archaeological and paleoenvironmental interests overlap. These karst depressions also served as favored locations for agriculture and settlement for thousands of years, adding a human dimension to the sedimentary archive.13Elsevier / Quaternary International. Karst depressions as geoarchaeological archives
Where Dolines Cluster and Why
Dolines are not distributed evenly across karst terrain. Their density and arrangement follow the underlying geology. Flat or gently sloping karst plateaus without active surface drainage tend to concentrate the most dolines, because water has nowhere to go but down. On the Miljevci plateau in Croatia, researchers using GIS analysis found that doline distribution is strongly clustered, with hotspots reaching 30 to 34 dolines per square kilometer. The density dropped as slope steepness and surface relief increased, confirming that gentle, plateau-like surfaces favor doline formation. Interestingly, the alignment of doline clusters along ancient river canyons suggested the presence of a former drainage network that once ran across the surface before being captured underground.14Acta Carsologica. GIS-based analysis of doline density on Miljevci karst plateau (Croatia)
This kind of spatial analysis has practical value. A high-density cluster of existing dolines signals that the bedrock beneath is actively dissolving and that new dolines are likely to form nearby. Planners and engineers can use doline density maps as a first-pass risk screen when siting roads, buildings, or utilities in karst regions.
Dolines Beneath the Sea
Dolines are not confined to dry land. Submarine sinkholes dot continental shelves and carbonate platforms around the world, formed during past periods of lower sea level when the rock was above water and exposed to rainwater dissolution. Some, however, may have formed underwater through entirely different chemistry.
On the Ganquan carbonate platform in the Xisha Islands of the South China Sea, researchers documented large submarine sinkholes whose inner walls display stalactite-like structures, flow marks, and corroded holes, all hallmarks of karst dissolution. Temperatures inside the sinkholes were about 2°C warmer than the open ocean at the same depth, hinting at a thermal influence. Based on seismic data, the researchers proposed that these features formed not from rainwater but from acidic hydrothermal fluids originating from magmatic activity below, rising along faults to dissolve the carbonate platform from within.15Journal of Marine Science and Engineering. First Documentation of Large Submarine Sinkholes on the Ganquan Carbonate Platform in the Xisha Islands, South China Sea This mechanism, acid from below rather than above, expands the range of environments where doline-like features can develop.
Blue holes in the Bahamas, Belize, and elsewhere are perhaps the best-known submarine dolines. These steep-walled, water-filled shafts formed as cave systems during glacial low-stands and were flooded as sea levels rose. They attract both recreational divers and scientists, since their deep, low-oxygen waters preserve sediment records with exceptional clarity.
Dolines as Analogues for Other Worlds
Collapse features resembling dolines have been identified on the surfaces of Mars and other rocky bodies. On Earth, certain unusual collapse structures offer diagnostic criteria for recognizing such features at a distance. The Nullarbor Plain in southern Australia, for instance, hosts a series of linear trenches formed by the subdued surface expression of deep cave collapse. Their muted surface morphology, where the collapse signature is partly hidden by overlying rock draping into the void, provides a useful template for interpreting similar linear depressions spotted on other planetary surfaces.16Communications Earth & Environment. Subdued surface expression of deep cave collapse If dissolution or void formation occurs beneath the surface of Mars, which has sulfate-rich and chloride-rich minerals that could dissolve under the right conditions, then doline-like landforms would be one of the expected surface signatures.