The Black Sea is very different from other seas and has a huge content of hydrogen sulphide, which makes it toxic, but this content is below about 100 meters.
This is because the sea is relatively new, having formed between 6 and 3 million years ago, before being a combination of lakes, swamps and farmland.
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The Black Sea was formed as a result of Mediterranean saltwater dumping into a huge Bosporus ravine, triggering an extinction event.
The toxins are largely the decomposition products of the plant and animal life that existed before the flood.
The upper layer of water is safe, being diluted by the fresh water of the rivers.
There is no marine life below approximately 200 meters. For example, if something metallic is suspended at this depth, it will corrode in no time.
