Is it true that 90 per cent of the Black Sea is toxic?

There is something true behind that figure, and it has no bearing on the water you swim in. Below roughly 150 to 200 metres the Black Sea carries no oxygen but hydrogen sulphide, and that accounts for around 90 per cent of the water volume. The stratification is stable and some 7,000 years old: heavy salt water from the Mediterranean below, lighter river water above, and the two do not mix. Swimming, diving and fishing all happen in the upper, oxygen-rich layer. Off the southern coast the question does not arise at all: the shelf is shallow, on the beaches from Sozopol to Sveti Vlas you stand in a few metres of water, and the deep layer lies far out. Bathing water quality is measured afresh every season by the environmental authority.