atoll

AT-all or ah-TOLL
A ring-shaped coral reef enclosing a shallow lagoon, with no central island. Atolls form when a volcanic island sinks slowly beneath the sea and the coral reef growing around its perimeter continues to build upward, eventually outliving the island that created it. The lagoon is the ghost of the mountain. Darwin figured this out in 1842, and he was right.
Etymology
Maldivian atholhu, a ring-shaped reef. The word entered English through early European contact with the Maldives.
geology Indigenous ocean water
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