Reef

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water (six fathoms or less beneath low water). Many reefs result from abiotic processes—deposition of sand, wave erosion planing down rock outcrops, and other natural processes—but the best-known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and calcareous algae.