The perception of the Black Sea being "dirty" is largely due to its unique geological and chemical makeup, which creates a phenomenon known as meromictic waters. It is the world's largest anoxic basin, meaning that below about 150-200 meters, there is absolutely no oxygen. This is because the sea is almost entirely enclosed, with only the narrow Bosphorus Strait allowing a small exchange with the Mediterranean. Because the top layer of fresher water (from rivers like the Danube) doesn't mix with the denser, saltier bottom layer, the deep water is saturated with hydrogen sulfide, which smells like rotten eggs and turns everything black—hence the name. Furthermore, because it is the drainage basin for much of Europe, it suffers from significant "eutrophication" caused by agricultural runoff. Fertilizers from the Danube, Dnieper, and Dniester rivers cause massive algae blooms that further deplete oxygen and can give the water a murky, green-brown appearance near the coast. While "polluted" in the industrial sense in some coastal zones, much of what makes it look "dirty" is actually a result of these natural, yet extreme, chemical imbalances.