An atoll ( /ˈæt.ɒl, -ɔːl, -oʊl, əˈtɒl, -ˈtɔːl, -ˈtoʊl/) is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can grow. Most of the approximately 440 atolls in the world are in the Pacific Ocean.