An atoll is a ring-shaped CORAL REEF, island, or chain of islands formed of CORAL. It typically encircles a lagoon and is composed of four components: a REEF, an ISLET, an ISLE, and CORAL. The REEF is a ridge of CORAL that lies beneath the surface of the ocean and is made up of calcium carbonate secreted by corals. An ISLET is a small island, usually no more than a few hundred meters in diameter. An ISLE is a larger island, typically measuring several kilometers in diameter. Finally, CORAL is a type of marine invertebrate that forms colonies of polyps and secretes calcium carbonate to form reefs.