Atoll

An atoll is a ring-shaped coral reef, island, or chain of islets enclosing a lagoon, usually formed as coral grows around a sinking volcanic island.

Fringing reef: A coral reef that grows close to a shoreline, with no broad lagoon separating it from land. It commonly forms the first reef stage around a volcanic island.
Coral reef: A marine ecosystem and structure built largely from the calcium-carbonate skeletons of corals. An atoll's rim is a coral reef built from accumulated skeletal material.
Bikini Atoll: A coral atoll in the Marshall Islands, used for United States nuclear weapons tests after World War II. Its reef and lagoon became the site of a consequential testing program.
Tuvalu: An island country in the Pacific composed of low-lying coral atolls and reef islands. Its communities face the consequences of living on very low atoll land.
Barrier reef: A coral reef separated from a coast by a lagoon or deeper water. It can develop as a volcanic island subsides and its reef continues growing upward.
Coral: A marine animal that often lives in colonies and secretes a calcium-carbonate skeleton. Reef-building corals construct much of an atoll's framework.
Enewetak Atoll: A coral atoll in the Marshall Islands where the United States conducted nuclear weapons tests. Its history shows how isolated atolls became locations for nuclear testing.
Kiribati: An island country in the central Pacific whose territory includes numerous coral atolls. Its dispersed atolls make transport, services, and climate adaptation difficult.
Darwin's theory of coral reef formation: Charles Darwin's explanation that fringing reefs, barrier reefs, and atolls form as volcanic islands subside. It explains the classic sequence that produces many atolls.
Lagoon: A shallow body of water separated from the open sea by a reef, barrier, or other landform. The enclosed lagoon is the defining water body inside an atoll's ring.

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