A microatoll is a circular colony of corals, near the surface of the ocean. Because the top is not in the water all the time, the corals at the top are dead. The corals which are not exposed to fresh air are still alive. The atoll will only grow in the areas which are in the water all the time. Microatolls may be up to in diameter. They are named for their resemblance to island atolls formed during the subsidence of volcanic islands, as originally suggested by Darwin (1842).