The term cave is given to a naturally formed underground cavity. The term usually implies that the opening is large enough for a person to enter and may indicate a length further than sunlight reaches. Some caves measure a few hundred feet while others extend for many miles and reach hundreds of feet in depth. Caves can be found in many parts of the world and in every state in the United States. Caves are geological features and form in a variety of ways.

The most common caves are solutional caves, often called limestone caves for the common type of soluble rock in which they form. This is the type of cave seen in the Journey into Amazing Caves movie in the canyon areas and the cave diving scenes. Both the terrestrial and underwater caves in the film are solutional or limestone caves. Most limestone caves form through the dissolution of soluble rocks by a weak, naturally occurring acid in groundwater. The minerals which make up the rock actually dissolve into groundwater, like sugar into hot tea, but much more slowly. The caves form as groundwater dissolves quantities of soluble rock by seeping along joints, faults and along bedding plains. In time the cracks enlarge to form cavern systems.

Sometimes these solutional caves are flooded with groundwater. This happens when the level of the groundwater, or watertable, is above the level of the cave passage. The underwater cave in the Journey into Amazing Caves movie was in this type of flooded cave passage. The entire cave can be filled with water, or only the cave passages below the water table will be filled with water.

Some limestone caves are decorated with cave formations called speleothems. These speleothems are formed as calcite is deposited on the surface of cave ceilings, walls and floors. The most commonly known speleothems are stalactities, which hang on the ceiling, and stalagmites, which grow up from the floor.

For more information on speleothems including some great pictures visit The Virtual Cave - http://www.goodearthgraphics.com/virtcave.html

Another type of cave in Journey into Amazing Caves movie film are ices caves. Ice caves can form under glaciers. Melting water running under the glaciers or through cracks in the glaciers can erode and melt tunnels or chambers in the ice. Ice caves can also be caves in rock which have a temperature below freezing all year. These caves have ice forming on the ceiling, walls and floors.

Examples of other cave types would include lava caves, sea caves and sandstone caves.