Coral rings, also known as atolls, are circular or horseshoe-shaped coral reefs that form around a central lagoon. They are typically found in tropical and subtropical waters and are made up of calcium carbonate secreted by corals. Atolls are formed over millions of years as the coral grows and builds up around the edges of an island or volcanic seamount. The coral rings are home to a variety of marine life, including fish, mollusks, and crustaceans.