In nautical terminology, a reef's a rock, sandbar, or other feature lying beneath the surface of the water (six fathoms or less at low water). Many reefs result from abiotic processes—deposition of sand, wave erosion planning 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. Artificial reefs such as shipwrecks are sometimes created to enhance phy… (More on Reef)