Caves are deep hollow places in the rocky sides of hills or cliffs. The larger ones are called caverns. Some caves were formed by the constant beating of the sea waves against the rocks. Some caves appeared under the surface of the earth. These are usually the old courses of underground streams, which have worn away the soft rocks such as limestone.
Still others were formed by the volcanic shifting of surface rocks or by eruption of hot lava. Some caves have openings through their roofs, called sink holes. Some caves have galleries in tiers or rows, one above the other. Then, there are those caves, which are formed in the limestone where water percolates down, carrying bits of lime, which builds into columns called stalagmite.
Below Caves info from Wikipedia (Read More):-
A cave or cavern is a natural underground void large enough for a human to enter. Some people suggest that the term cave should only apply to cavities that have some part that does not receive daylight; however, in popular usage, the term includes smaller spaces like sea caves, rock shelters, and grottos.
Speleology is the science of exploration and study of all aspects of caves and the environment which surrounds the caves. Exploring a cave for recreation or science may be called caving, potholing, or, in Canada and the United States, spelunking (see Caving).
http://en.wikipedia.org/wiki/Caves
