It is the only double sea on the earth.

It is the only double sea on the earth.
The river flows into a large amount of fresh water, which makes the upper layer contain much less salinity than the lower layer. At the junction of the upper and lower layers, the water depth is between 100 and 150 meters. It can only be mixed for about 1,000 years.
[Hypoxia Black]
Because the Black Sea cannot communicate between deep and shallow water, it presents a severe hypoxia state below 220 meters, and only anaerobic microorganisms can survive.
Hydrogen sulfide released by anaerobic microorganism metabolism interacts with metal ions in the water to produce black sulfide, which causes deep seawater to appear black.