As the groundwater dissolves limestone, it creates voids and removes the rock that supports the surface. At the ground surface, sinkholes form where the ground has subsided underneath Some valleys have no "exit" where a stream leads to a river. Instead, the water drains into the sinkhole, enters the cave, and then exits the cave at a spring before reaching the river.
Some surface streams also lose water to a subsurface conduit, and can even appear to dry up before reaching a river. A classic "losing stream" is Sinking Creek in Giles County. Most of the water in the stream sinks underground about a mile before the stream reaches the New River, leaving a rocky streambed at the mouth of the creek that is filled with water only during storms.
The term karst topography describes the landforms in a region with a large number of caves, sinkholes, and losing streams. USGS quad maps use hatched contour lines to indicate sinkholes, and cavers carefully explore karst areas in hopes of discovering an entrance to a previously-undiscovered cave.
karst topography (colored dark red) is common near Pembroke (Giles County)
Source: US Geological Survey (USGS), Map showing surficial and generalized bedrock geology and accompanying side-looking airborne radar image of the Radford 30' X 60' quadrangle, Virginia and West Virginia
Nearly all caves show signs of rockfalls from ceilings, and a cave room may grow so large that it collapses. Mother Nature does not excavate rooms according to the same engineering designs used by coal miners, leaving pillars to support the roof.
Natural Tunnel and Natural Bridge show what happens when just a portion of the roof collapses. Natural Chimneys was formed by erosion of the limestone ceiling and one side of a former cave.
The general rule is that limestone caves are located near the surface, in the top 1,000 feet. The acidic water is concentrated there. By the time the surface water reaches a deeper depth, it has been neutralized and is no longer able to dissolve calcium carbonate. In New Mexico, however, some very deep caves have been formed by acid fumes rising up from underground gas deposits, so there is an exception to every rule.