Caves are hollow spaces carved into hillsides or underground by the action of water and the wind. On coasts, pounding waves sometimes hollow out caves in the bases of cliffs.
Caves are most common in limestone rock. Rainwater, which contains a weak acid, dissolves the soft limestone and seeps into cracks, eventually carving out channels that widen into tunnels and caves. Caves are also found in glaciers and in cooled volcanic lava.