Coral reefs are the ocean’s richest ecosystem in terms of biodiversity and productivity. They are restricted to tropical waters, where conditions of salinity, temperature and sedimentation are suitable. Where they grow, their main benthic organisms deposit substantial limestone skeletons, such that they effectively make their own habitat which sustains their dynamic nature and supports the wide range of species which inhabit them. Reefs grow to the low tide level, thus providing a breakwater, but the richest parts lie 5–20 metres below the surface, an area where light is still sufficient but where sediment and turbulence are not severe. Reefs may occur as narrow fringing reefs bordering a continental coast, as huge offshore barrier reefs or as series of atolls that support entire nations; the biogenic nature of corals is enormously important to mankind.