An atoll (/ˈætɒl/, /ˈætɔːl/, /ˈætoʊl/, /əˈtɒl/, /əˈtɔːl/ or /əˈtoʊl/), sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands/cays on the rim. 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.