Atoll << AT ol >> is a circular ring of coral in the open sea, built up on a sunken bank, or formed on the crater of a volcano that has sunk below the surface of the sea. A thin layer of soil generally lodges on the reef, and vegetation springs up. Tropical plants thrive on the atoll, and it becomes a coral island. Atolls enclose shallow pools or lagoons. One or more channels usually connect the lagoon of most atolls to the surrounding sea, on the windward side. Atolls are found chiefly in the Pacific Ocean. See also Coral; Coral reef.
