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
Open-water diving water, with various constraints regarding the conditions, and particularly that their competence is limited to diving in open water of with free access to the surface. Open-water diving In underwater diving, open water is unrestricted water such as a sea, lake or flooded quarries. It is the opposite of confined water (diving) (usually a swimming pool) where diver training takes place. Open water also means the diver has direct vertical access to the surface of the water in contact with the Earth's atmosphere. Open water diving implies that if a problem arises, the diver can directly ascend vertically to the