When limestone interacts with underground water, the water dissolves the limestone to form karst topography - an amalgamation of caves, underground channels, and a rough and bumpy ground surface. Karst topography is named for the Kras plateau region of eastern Italy and western Slovenia (Kras is Karst in German for "barren land").
The underground water of karst topography carves our impressive channels and caves that are susceptible to collapse from the surface. When enough limestone is eroded from underground, a sinkhole (also called a doline) may develop. Sinkholes are depressions that form when a portion of the lithosphere below is eroded away.
Sinkholes can range in size from a few feet or meters to over 100 meters (300 feet) deep. They've been known to "swallow" cars, homes, businesses, and other structures. Sinkholes are common in Florida where they're often caused by the loss of groundwater from pumping.
A sinkhole can even collapse through the roof of an underground cavern and form what's known as a collapse sinkhole, which can become a portal into a deep underground cavern.
While there are caverns located around the world, not all have been explored. Many still elude spelunkers as there is no opening to the cave from the earth's surface.
Inside karst caves, one might find a wide range of speleothems - structures created by the deposition of slowly dripping calcium carbonate solutions. Dripstones provide the point where slowly dripping water turns into stalactites (those structures which hang from the ceilings of caverns), over thousands of years which drip onto the ground, slowly forming stalagmites. When stalactites and stalagmites meet, they forum cohesive columns of rock. Tourists flock to caverns where beautiful displays of stalactites, stalagmites, columns, and other stunning images of karst topography can be seen.
Karst topography forms the world's longest cave system - the Mammoth Cave system of Kentucky is over 350 miles (560 km) long. Karst topography can also be found extensively in the Shan Plateau of China, Nullarbor Region of Australia, the Atlas Mountains of northern Africa, the Appalachian Mountains of the U.S., Belo Horizonte of Brazil, and the Carpathian Basin of Southern Europe.
See this topographic map of Mitchell, Indiana for an impressive example of karst topography (remember that contour lines with tick marks are depressions). On this map, each contour line represents 10 feet (3 meters).