Karst is a German name for an unusual and distinct limestone terrain in Slovenia, called Kras. The karst region in Slovenia, located just north of the Adriatic Sea, is an area of barren, white, fretted rock. The main feature of a karst region is the absence of surface water flow. Rainfall and surface waters (streams, for example) disappear into a drainage system produced in karst areas. Another feature is the lack of topsoil or vegetation. In geology, the term karst topography is used to describe areas similar to that found in Kras. The most remarkable feature of karst regions is the formation of caves.
Karst landscapes develop where the bedrock is comprised of an extremely soluble calcium carbonate rock, for example: limestone, gypsum, or dolomite. Limestone is the most soluble calcium carbonate rock. Consequently, most karst regions develop in areas where the bedrock is limestone. Karst regions occur mainly in the great sedimentary basins. The United States contains the most extensive karst region of the world. The Mammoth cave system is located in this area. Other extensive karst regions can be found in southern France, southern China, Central America, Turkey, Ireland, and England.
Karst regions are formed when there is a chemical reaction between the groundwater and the bedrock. As rain, streams, and rivers flow over the earth's surface, the water mixes with the carbon dioxide that naturally exists in air, and the soil becomes acidic and corrodes the calcium carbonate rock. The carbonate solution seeps into fissures, fractures, crevices, and other depressions in the rock. Sinkholes develop and the fissures and crevices widen and lengthen. As the openings get larger, the amount of water that can enter increases. The surface tension decreases, allowing the water to enter faster and more easily. Eventually, an underground drainage system develops. The bedrock is often hundreds of feet thick, extending from near the earth's surface to below the water table. Solution caves often develop in karst regions. Caves develop by an extensive enlargement and erosion of the underground drainage structure into a system of connecting passageways.
There are many variations of karst landscape, often described in terms of a particular landform. The predominant landforms are called fluviokarst, doline karst, cone and tower karst, and pavement karst. Some karst regions were etched during the Ice Age and may appear barren and very weathered (pavement karst). Other karst areas appear as dry valleys for part of the year and after seasonal floods, as a lake (one example of fluviokarst). In tropical areas, karst regions can be covered with forests or other thick vegetation. Sometimes, the underground drainage structure collapses, leaving odd formations such as natural bridges and sinkholes (doline karst). Tall, jagged limestone peaks are another variation (cone or tower karst).