A band of underground water-soluble rock (gypsum, or more commonly, limestone) is prone to dissolution by natural groundwater.
Water from the surface or flowing underground through fissures dissolves water-soluble rock such as gypsum and, more commonly, limestone. This results in the eventual creation of karst scenery. These may form a network of caves, micro-caves and enlarged fissures. They represent periods of sea level drop in prehistoric times, and may contain fossils and/or speleothems.
Surface access to karst caves is not always guaranteed, as it depends on the geography of the area and the nature in which the karst was formed. These caves tend to be regularly flooded so a canoe or raft is advisable, and the differing size of caves means that the water may reach very close to the ceiling in some. Occasionally the ground above these caves may collapse into a sinkhole, usually after a large storm. Water freezing in cracks in the rock during winter expands, which may also cause weaknesses in the limestone, leading to collapses.
Missouri has a large portion of karst topography in the US due to its abundance of limestone. Of particular interest are the numerous caves of karst origin under and around the City of St. Louis, MO. Another area of well-developed karst landscapes is southern China. The Niagra Escarpment in Southern Ontario, Canada is also known for its karst landscapes.