An atoll barrier is a type of CORAL REEF that is formed in a ring-like shape around a central lagoon. It is made up of a REEF, an ISLET, corals, and a BIKINI. The REEF is a large area of rock or CORAL that lies beneath the surface of the water. An ISLET is a small island, usually made of sand or CORAL. Corals are colonies of tiny animals that live in the ocean and create a hard, stony skeleton. BIKINI is a type of CORAL that grows in shallow waters.