If you are referring to the crossword puzzle hint, try Atoll.
A circular coral island that encloses a lagoon is called an atoll. Atolls are typically formed from coral reefs that grow around the rim of a submerged volcanic island.
atoll
An atoll is a ring-shaped coral reef island that encircles a lagoon partially or completely. Atolls are typically found in the Pacific Ocean and are formed from the remnants of volcanic islands that have submerged over time. The circular shape of atolls is a result of coral growth around the rim of a sinking volcanic island.
138.23m approx. 2*pi*r
Each island is surrounded by a reef enclosing a shallow lagoon.
staten island
An encircling coral reef or sandbar can create a shallower area of water near an island, called a lagoon.
An island with a central lagoon is called an atoll. Kwajalein in the Indian Ocean is a very good example.
A Barrier Reef. The barrier reefs grow parallel to the shore of an island and are separated from it by a deep lagoon.
An atoll is a ring-shaped coral island or string of islands that surround a lagoon. Atolls typically form from volcanic islands that sink into the ocean, leaving a coral reef formation around the edge of the submerged island. The lagoon in the center of an atoll is often shallow and can support diverse marine life.
Poveglia Island is located in the lagoon of Venice, Italy.
barrier reef