An atoll is a ring-shaped coral island, or chain of islands, that surrounds a lagoon, usually in warm ocean waters.
It comes into English from Dhivehi (Maldivian) *atholhu*, meaning 'lagoon island'. The word was adopted by European sailors exploring the Indian Ocean.
Atolls are like living architecture: tiny coral animals slowly build massive ring-shaped structures over thousands of years. Many atolls are the ghost outlines of old volcanoes, with the rock gone and only the coral ring remaining.
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