A sinkhole is a topographic depression or hole formed when limestone bedrock is dissolved by groundwater. For examples and pictures of sinkholes, read on here. Sinkholes are a defining feature of karst topography, a type of terrain characterized by barren rocky ground, the absence of surface stream and lakes, and the presence of caves and underground rivers. The term is named for the Karst region in Slovenia, but now is used to mean all areas with similar features, found widely around the world. In the US, areas of the Mid-West, Kentucky and Florida are all considered Karst regions.

For maps of sinkhole regions, the USGS has a map of the United State's sinkhole hotspots, evaporite bedrock and volcanic bedrock here, and Why Map has a world Karst aquifer map here. For information on how to deal with sinkholes should you ever encounter one, this article includes suggestions for small underground cavity remediation.