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1620s, atollon, from Malayam atolu "reef," probably from adal "closing, uniting." Popularized in present form by Darwin's writings.
A coral island or series of coral islands forming a ring that nearly or entirely encloses a shallow lagoon. Atolls are surrounded by deep ocean water and range in diameter from about 1 km (0.62 mi) to over 100 km (62 mi). They are especially common in the western and central Pacific Ocean and are believed to form along the fringes of underwater volcanoes. Compare barrier reef, fringing reef.
A coral island that surrounds a lagoon. (See coral reef.)