Coral itself may constitute less than half the material found in a coral reef, which is a wave-resistant underwater ridge or mound built of fragments of coral, coral sands, and solid limestone. The reef may be at or slightly below sea level. Three forms of coral reefs are defined: Fringing reefs are located close to shore, normally separated from land by only shallow water. Barrier reefs lie farther offshore, separated from land by lagoons more than 30 feet (10 m) deep. Atolls are found far offshore and are made up of a ring-shaped reef enclosing a circular lagoon. Each of the four stamps of this set shows one type of coral and one species of fish found in coral reefs.