Coral reef A fringing reef forms around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the level of the underlying earth allows, the corals grow around the coast
Zhongzhou Reef Zhongzhou Reef Zhongzhou Reef also known as Ban Than Reef and Centre Cay (; ) is a small coral reef on the north edge of the Tizard Bank in the Spratly Islands, South China Sea. It lies east of ROC-occupied Taiping Island (Itu Aba), west of Vietnamese-occupied Sand Cay, and has an exposed area of approximately during high tide, and during low tide. It consists of an accumulation of seashells, sand, coral reefs and debris which forms a circular coral reef plate, surrounding the reef under the water, with a diameter of approximately . The reef is currently controlled by