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NASA. Via Wikipedia
An atoll is a roughly circular group of small islands around a central lagoon. It forms from a volcanic island that is eroded to sea level by wave action. Subsequent subsidence of the island produces a coral reef around the edges of the submerged mountaintop, and wind and surf pile up coral fragments to form the islets.
Because of the small size of their islands, many atolls lack sources of fresh water. However, the central lagoon often forms an excellent natural harbor. There are usually several passes into the central lagoon, corresponding to river mouths of the original volcanic island, whose fresh water inhibited coral growth.
Most of the island groups of the central Pacific, including the Gilberts, Marshalls, and many of the Caroline Islands, consisted of chains of atolls. The small size of the islands meant that amphibious assaults on them were set piece frontal assaults with little opportunity for maneuver.
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