A ring-shaped reef is a type of ATOLL, which is a coral reef that forms a circular or horseshoe-shaped island around a lagoon. Atolls are typically found in the tropical regions of the world and are formed over millions of years as coral grows and dies, leaving behind a ring of coral. The lagoon in the center of the ATOLL is usually shallow and filled with marine life, making it a popular destination for snorkelers and divers.