Microatoll

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).
They have been used to monitor sea levels, in response to global warming. They have also been used to quantify and date changes in sea level in seismically active areas. Using microatolls, scientists were able to provide information about the water temperature at the surface of the sea.