A cave, or cavern, is an underground chamber that opens to the surface and is often large enough for a person to enter. A network of caves connected by passages is called a cave system. An example is Carlsbad Caverns, a cave system in New Mexico, has at least 34 kilometers (21 mi) of passages. One of the caves in the system is so big that It could accommodate 11 football fields.
Most caves form from the dissolution of rock, such as limestone, marble, or dolomite, that dissolves in a mixture of rainwater and carbon dioxide, Rainwater absorbs carbon dioxide from the air, forming a very weak acid (see pH). This acidic water seeps down into tiny cracks. In rock below the soil. cracks and spaces in the rock fill with water until the area is saturated. The surface of the saturated area is called the water table. Below the water table, water flows horizontally and eventually drains into streams. As it flows, the acidic water dissolves surrounding rock and carries it away.
Over thousands of years, the flowing water hollows out channels, or tunnels, in the rock. In time, the water table may drop below the level of a tunnel, Then the tunnel fills with air and becomes a cave,
Water that trickles into caves contains dissolved limestone, or calcium carbonate, The water evaporates, leaving minerals that build up into formations called dripstone, Minerals in the water determine a formation's color, and the way the drops flow affects its shape. Water dripping from the roof of a cave deposits calcium carbonate that may form stalactites, icicle-shaped pieces of dripstone that hang from the ceiling, When water drips onto a cave floor the minerals it deposits may build up into spires called stalagmites.
The study of caves is known as speleology. People who explore caves are called cavers; or spelunkers.
There are some 17,000 known caves In the United States. Many more remain to be explored. More than 100 caves are open to the public, and some 15 have been designated as national parks or monuments,
The formation of caves makes a region vulnerable to subsidence