The marine habitat in the middle of an atoll, protected from the open ocean by the sturdy coral reef, is called a lagoon. The shallow lagoon of the Mataiva Atoll, in French Polynesia, stands out as a striking blue in this satellite image. The turquoise color and white ridges are caused by decaying coral reefs.

Photograph courtesy NASA

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atoll
atoll Encyclopedic Entry

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Encyclopedic entry. An atoll is a ring-shaped coral reef, island, or series of islets. The atoll surrounds a body of water called a lagoon.