The bottom of the Black Sea is considered toxic and essentially "dead" because it is the world's largest anoxic (oxygen-depleted) water basin. Below a depth of about 150 to 200 meters, the water contains no dissolved oxygen, which is a result of the sea's unique geographical structure and restricted water exchange with the Mediterranean through the narrow Bosphorus Strait. Because the upper layer of fresher, less dense water (fed by rivers like the Danube) does not mix with the saltier, denser lower layer, the deep water becomes stagnant. In this oxygen-free environment, specialized bacteria decompose organic matter and produce a high-fidelity concentration of hydrogen sulfide (H2S), a poisonous gas that smells like rotten eggs. This toxic layer makes up about 90% of the sea's total volume, meaning that except for the surface layer, the Black Sea is unable to support most forms of marine life, preserving ancient shipwrecks in a "High-Fidelity" state for centuries due to the lack of oxygen and wood-boring organisms.