A ring-shaped coral reef or island that encircles a lagoon, typically found in tropical ocean waters. They form when coral grows on the rim of a submerged volcanic island.
From Dhivehi (Maldivian) atholhu meaning 'island group'. The word entered English through early maritime explorers in the Indian Ocean region during the 16th-17th centuries. Charles Darwin later used the term in his coral reef formation theory.
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