Karst

Karst is a geological system of rocks where water has eroded (dissolved) the bedrock. If it is very extensive, karst can be a geographical or topographical feature. It is classed as a badland if it difficult to travel through the environment, which it often is.
Almost always, karst is formed in carbonate rocks, such as limestone or dolomite. Since carbonate sedimentary rocks are very common, karst areas occur all over the world.
Many karst regions display distinctive surface features, with sinkholes being the most common. However, karst surface features may be absent where the soluble rock is "mantled" (covered), such as by a non-soluble rock strata on top of the carbonate strata. Some karst regions include thousands of caves, and the water is generally underground.
The world's largest karst area is the Nullarbor Plain, part of the area of flat, almost treeless, dry country of southern Australia. It is on the Great Australian Bight coast with the Great Victoria Desert to its north. It is the world's largest single exposure of limestone bedrock, and occupies an area of about . At its widest point, it stretches about from east to west across the border region between South Australia and Western Australia.
Sinkholes.
Many parts of the world suffer from sinkholes which form when karsts are underground. Beneath the flat surface of the land in Florida, Guatemala City and elsewhere are vast systems of limestone. Water running through the limestone is dissolving the limestone away, leaving huge caverns. Without warning, huge holes appear in the surface, causing great damage. Practically the whole of Florida is sitting on a gigantic karst system, as are several countries in Central America. The area round Tampa is especially at risk, and there is a huge karst system under Mexico City. According to a senior geochemist, there are as many as 15,000 sinkholes in Illinois.
The world's deepest sinkhole is China's Xiaozhai Tienkeng at 2172ft (662m). 
The Qattara Depression in Egypt is part of the Libyan Desert. It is roughly 50 miles (80km) by 75 miles (121km) in surface area. It is the world's largest natural sinkhole. It lies below sea level and is covered with salt pans, sand dunes and salt marshes.