The Black Sea sits on continental plates which have subsided. It is a geologic basin, and therefore is a genuine inland sea. Its maximum depth is about 2000 metres. The Black Sea is the world’s largest basin where the deep waters do not mix with the upper layers of water that receive oxygen from the atmosphere. As a result, over 90% of the deeper Black Sea volume is anoxic water. The upper layers are generally cooler, less dense and less salty than the deeper waters, as they are fed by large river systems, whereas the deep waters come from the warm, salty waters of the Mediterranean. 