An Atoll is a coral island that encircles a lagoon, either partially or completely. Atolls are formed through the extinction of volcanic islands. As the island and the ocean floor subside, corral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef. As the subsiding continues, the fringing reef becomes the larger barrier reef farther from the shore, with a bigger and deeper lagoon inside. Eventually, the island sinks below sea level and the barrier reef becomes an atoll, enclosing the open lagoon.