Neversink Pit, a wet limestone sinkhole in Alabama seen above in 1998, is about 50 feet (15 meters) deep and houses a rare species of fern. The sinkhole was bought in the 1990s by a group of cavers to preserve it for future generations.
Karst is the geologic term for landscapes formed mainly by the dissolving of limestone or dolomite bedrock. In the United States, karst underlies parts of Missouri, Arkansas, Kentucky, Tennessee, northern Alabama, Texas, and most of Florida (see a USGS karst map).
Such areas are marked by sinking streams, subterranean drainage, large springs, caves—and, of course, sinkholes.