Atoll

An atoll (/ˈætɒl/ or /əˈtɒl/)^[1] is a ring-shaped coral reef includin a coral rim that encircles a lagoon pairtially or completely. Thare mey be coral islands/cays on the coral rim.^{[2](p60) [3]}

Satellite pictur o the Atafu atoll in Tokelau in the Paceefic Ocean.