Features of an atoll are: They thrive in warm tropical and subtropical waters. Most are found in the Pacific Ocean. They are products of the growth of topical sea organisms. They are usually round and formed around a lagoon.
An atoll is formed first as a reef that fringes a volcanic island. As the island sinks (after the volcanic activity has ceased and the crust has cooled, becoming denser), the reef continues to build upward, eventually ending up as a ring-shaped structure.
A lagoon forms when a body of water is enclosed behind a barrier reef. It can also form together with the formation of an atoll, it being the body of water in the middle of the atoll. A lagoon is a body of comparatively shallow salt or brackish water separated from the deeper sea.