Cave
Cave, an underground cavity in the earth's crust. Cavern is a synonymous word, but is sometimes used to indicate a very large cave. The study of caves is called speleology. Spelunking is the hobby of exploring caves.
Most caves are formed by the dissolving action of water; such caves are sometimes called solution caves. Other caves owe their origin to such phenomena as escaping volcanic lava and gas or the action of waves along a seacoast.
Solution caves usually occur in limestone and are created by slightly acidic underground water slowly dissolving the rock along cracks. Gradually, over many thousands of years, numerous passages and chambers are formed. Often, the passages extend for great distances and are intricately interconnected. Frequently, there are multiple levels. Underground rivers, pools, and waterfalls are found in many caves.
Various mineral formations and deposits, collectively called speleothems, are usually associated with caves. Most of these formations are of travertine or gypsum and are caused by water Seeping, dripping, or flowing into the cave from above. Dripping from a cave ceiling, the water deposits iciclelike stalactites of limestone, and gradually builds up pillars, or stalagmites, from the floor. The stalactites and stalagmites may be colorfully tinted by minerals in the water. Some caverns are decorated with draperies millions of years old.
Most caves are damp and cool; some are dry with virtually no sign of dripping or running water. Temperatures remain rather constant throughout the year, even in areas of great seasonal extremes. Cave entrances may be either downward from the surface of the earth or horizontally from the side of a hill or cliff. In some areas sinks, or sinkholes, occur at the surface of the earth, indicating the collapse of cave ceilings. If sinks and caves are numerous over a broad area, the terrain is called karst topography, or karst.
Caves are found throughout the world. In the United States more than 100 are open to visitors. Some are in national parks and monuments; others are the focus of state parks. Among those that are in national parks are two of the country's most widely known caves—Mammoth Cave, in Kentucky, and Carlsbad Caverns, in New Mexico.
Caves are associated with legend and superstition. At times they were thought to be passages to the underworld. A widespread legend is one in which a national hero sleeps in a cave, to awaken years later and rescue his people. Caves were used as temples in ancient Greece, Egypt, and India. The Greek oracle of Delphi was said to live in a cave, as did Roman nymphs and sibyls.