Reef

A reef is a ridge or shoal of rock, coral or similarly stable material lying beneath the surface of a body of water, often near enough to the surface to be a hazard to navigation, though not every definition requires shallow depth. Reefs form in three ways: abiotic reefs arise from non-living processes such as sand deposition or waves planing down a rock outcrop, biotic reefs such as tropical coral reefs are built by living corals and coralline algae, and artificial reefs are created by people, deliberately or through shipwrecks and construction debris, and can shelter marine life that might otherwise decline. Reefs are classified by their origin, location, depth and shape, and they range from tropical coral fringing reefs to rocky reefs in temperate intertidal zones; the Great Barrier Reef off Australia, the largest coral reef system on Earth, stretches more than 2,300 kilometers.