An ATOLL is a type of coral reef that surrounds a lagoon. It is typically formed by a series of coral islands that encircle a central lagoon. The word 'ATOLL' comes from the Dhivehi language, which is spoken in the Maldives, and it means 'a ring-shaped reef or island'. Atolls are found in tropical oceans around the world, and they provide important habitats for many species of fish and other marine life.