Coral formations are geological structures made up of coral colonies and their calcium carbonate skeletons. The two most common types of coral formations are reefs and atolls. Reefs are large, underwater structures composed of coral colonies and other organisms that form a barrier between the shoreline and the open ocean. Atolls are circular or horseshoe-shaped coral formations that surround a lagoon. They are typically found in tropical and subtropical waters. Both reefs and atolls provide habitats for a variety of marine life and are important to the health of the ocean ecosystem.