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Sinkholes, also known as sinks, dolines (in the Slovene language doline means valleys), and cenotes, are formed by the collapse of cave roofs and are a feature of landscapes that are based on limestone bedrock. The result is a depression in the surface topography. This may range anywhere from a small, gentle earth-lined depression, to a large, cliff-lined chasm. Most often there is a small area of rock exposure near or at the bottom of a sinkhole, and a patent opening into the cave below may or may not be visible. In the case of exceptionally large sinkholes, such as Cedar Sink at Mammoth Cave National Park, there may actually be a stream or river flowing into the bottom of the sink from one side and out the other side.
Sinkholes often form in low areas where they form drainage outlets for an closed local surface drainage basin. They may also form in currently high and dry locations. Florida has been known for having frequent sinkholes, especially in the central part of the state.
Sinkholes are usually but not always linked with a karst landscape. Karst represents a set of surface features that are characteristic of limestone under the soil. In many such regions, there may be hundreds or even thousands of sinkholes in a small area so that the surface as seen from the air looks pock-marked. Often in such areas there are few or no flowing streams on the surface because the drainage is all sub-surface.
Sinkholes have for centuries been used as disposal sites for various forms of waste. An unfortunate consequence has been the pollution of groundwater resources, with serious health implications in such areas.
Sinkholes also form from human activity, such as the rare but still occasional collapse of abandoned mines in places like West Virginia. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater and subsurface fluids.
Natural sinkholes can be very deep, offering extremely challenging conditions for adventurous and experienced divers. Two of the most spectacular are the Zacaton cenote in Mexico and the Boesmansgat sinkhole in South Africa.