An atoll is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely.

The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water.

The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs.

For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upwards and outwards to replace the lost height.

The distribution of atolls around the globe is instructive: most of the world’s atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles).

The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.

Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are only found in the tropics and subtropics.

The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the North-western Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58′ S, and nearby Middleton Reef at 29°29′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory.