Karst is the land of caves, sinkholes, and sinking streams and is formed when soluble rocks are dissolved by water. Karst occurs worldwide and in the United States, approximately 20 percent of the land surface is karst.
Karst distribution in the United States (map data compiled from the USGS karst map and database, and the USGS Groundwater Atlas of the United States)
Karst terrain is where rivers and streams can sink directly into the ground:
Coyote Creek goes underground at the Calaveras Natural Bridge in California. In this unusual case, travertine deposits from springs in the river canyon had bridged the creek and been undercut over time by erosion of the marble streambed beneath it. This photo was the 2022 1st Place Winner in KWI’s photo contest (Photographer: Dave Bunnell, Location: Coyote Creek, Calaveras Natural Bridge, California, USA).