In physical geography, a sinkhole is a feature of landscapes based on limestone bedrock, formed by the collapse of cave roofs. 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 a surface drainage basin. They may also form in high and dry locations.
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 earth 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 solid and liquid wastes. An unfortunate consequence has been terrible, even toxic, pollution of underground water resources which has had serious health implications in such areas.