An atoll is a horseshoe-shaped or ring-shaped coral reef enclosing a lagoon.
Atolls are formed by the skeletons of sea creatures called corals. These corals live in colonies, and their skeletons collect over the course of years to form a reef. Then sand collects on the reef, and vegetation eventually follows.
The largest atoll in the world is Christmas Island, which is located near the Equator in the Pacific Ocean. Part of the nation of Kiribati, Christmas Island is an atoll with an area of 184 square miles.
Including the water area enclosed by a reef, the largest atoll in the world is Kwajalein, in the Marshall Islands in the Pacific.
Kwajalein consists of an atoll with a total length of 175 miles, enclosing a lagoon with an area of some 1,100 square miles.