An atoll is an island made of coral that sits in a ring, with a quiet, shallow sea — a lagoon — in the middle. From above it looks like a necklace of land around a pool. Warm oceans grow them; the Maldives and many Pacific islands are famous atolls. Though actually, the middle was not always water. Many atolls began as volcano islands. Coral built a ring around the island; the volcano slowly sank; the coral kept growing up, and you were left with a ring and a lagoon. It is more specific than island, and it is not the same as a reef that never becomes a ring of land you can stand on.

環礁（一圈珊瑚島，中間是潟湖）。