Ring-shaped reefs, also known as atolls, are a type of coral reef that form a circular or horseshoe-shaped island around a central lagoon. Atolls are typically found in the warm, shallow waters of the tropics and subtropics, and are formed over millions of years by the accumulation of coral debris and sediment. They provide important habitats for a variety of marine life, including fish, turtles, and seabirds.