During the expedition led by N.I. Andrusov, in 1980, it became known that starting from about a depth of 200 meters, the waters of the Black Sea are lifeless due to the presence of hydrogen sulfide in them. The expedition in 1891 confirmed and supplemented these data. Scientists explained this phenomenon as follows, let's look at the Black Sea as a reservoir of seawater coming from outside. As early as 9 thousand years ago, the Black Sea, like the Caspian Sea, was isolated from the world's oceans. Global warming has raised ocean levels and saline Mediterranean waters poured into the Black Sea basin, displacing lighter freshwater to the surface. In the course of further events, three layers were formed: thermocline, halocline, pycnocline. Almost the entire volume of the Black Sea is contained in the pycnocline, which feeds on salt water through the Bosphorus Strait. As a result, the bulk of the Black Sea is practically isolated and lacking oxygen supply.An oxygen-free environment served as one of the conditions for the emergence of hydrogen sulfide in the depths of the Black Sea. So, the Black Sea can be called a large generator and storage of hydrogen sulfide. The concentration of hydrogen sulfide increases with depth. Thus, the total reserves of hydrogen sulfide are estimated at tens of billions of tons with an annual increase of 4-9 million tons. Scientists warn that with the further accumulation of hydrogen sulfide in the water of the Black Sea, it will lead to a global catastrophe, and to the death of all life in the water.