An atoll is an island formed by a ring-shaped coral reef encircling a lagoon. The word comes from the language of the Maldives, an island chain in the Indian Ocean famous for gorgeous beaches and coral reefs.
The word atoll appeared in English in the early 17th century, when the British and the Dutch were vying for control of Indian Ocean trade. You may have heard of Bikini Atoll, in the South Pacific — where the U.S. did extensive nuclear testing in the 1940s and 1950s. Its people were moved to neighboring atolls but haven’t been able to return because of lingering radiation. Even so, Bikini Atoll has become a destination for divers interested in WWII shipwrecks.