When underground water erodes sedimentary bedrock, usually limestone, the action creates karst topography. Karst landscapes characteristically have a rough and bumpy surface and feature caves, sinkholes and disappearing springs and streams.
Karst topography is named after the Kras Plateau of northeastern Italy and western Slovenia. In the United States, Florida, Kentucky, Indiana and the Appalachian mountains feature sizable areas of karst. Other karst areas include the Nullabor region of in Australia, the Shan Plateau in China, the Atlas Mountains in North Africa, the Carpathian Basin of eastern Europe and Belo Horizonte in Brazil.
Caves in karst country often have striking features such as stalactites, stalagmites, columns and areas of flowstone. These rock forms are the result of precipitation of dissolved calcium carbonate from water as it seeps through the caves. When a cave collapses from the surface, it becomes a sinkhole. Collapse is likelier when groundwater levels drop, leaving a cave roof to support its own weight. An excellent example of a sinkhole is the 120 foot-deep Devil's Millhopper in Gainesville, Florida.
Springs and streams in karst country are often linked to underground water through a complex system of "swallets," places where water is sucked underground and then rises and returns to the surface. These water systems also have "siphons," which are structures that channel water back into the underlying aquifer.