Barrier reefs are reefs that are separated from land by a lagoon. These reefs grow parallel to the coast and are large and continuous. These features are followed by another coral terrace and a vertical drop into deeper waters. The third type of coral reefs are atolls. Atolls are annular reefs that develop at or near the surface of the sea when islands that are surrounded by reefs subside.
Atolls separate a central lagoon and are circular or sub-circular.