The Black Sea looks, at first glance, like any other large, glittering sea. It has beaches, fishing boats, dolphins and busy ports.
But below the waves, it becomes much stranger. Dive deep enough and you reach a vast underwater realm where oxygen largely disappears, most animals cannot survive, and ancient wrecks can remain eerily well preserved.
Where is the Black Sea?
The Black Sea sits between south-eastern Europe and western Asia. Six countries have coastlines on it: Bulgaria, Romania, Ukraine, Russia, Georgia and Turkey. It is linked to the Mediterranean Sea through the Bosphorus Strait, which leads to the Sea of Marmara and then the Dardanelles Strait, and is fed by several major rivers, including the Danube, Dnieper and Don.
Why is it called the Black Sea?
Despite the name, the Black Sea is not black. One theory is that the name comes from an ancient system that linked colours with compass directions. In that scheme, black or dark could mean north – so to people living south of it, the Black Sea may have been the “northern sea”. The idea is debated, but it helps explain why a sea that is not actually black might have acquired such a dramatic name.
- It's as long as a basketball court, as tall as a giraffe and so huge it can be seen from space
- Deep enough to swallow skyscrapers and filled with deadly toxic gases, it plunges so far into Earth even robots have never reached the bottom
How big and deep is the Black Sea?
The Black Sea is enormous. It covers about 436,000km2 – roughly twice the size of the United Kingdom – and stretches for more than 1,000km from east to west. Its deepest point reaches a little over 2,200m, making it far deeper than the North Sea.
Yet its most extraordinary feature is not its size, but its chemistry.
- It’s deep enough to hide Mount Kilimanjaro, contains bone-eating worms and is the world’s largest habitat
- It's big enough to swallow cities, deep enough to hide skyscrapers and inside lies an ancient, thriving 'lost world' cut off from everything above
What makes the Black Sea so weird?
The Black Sea is often described as the world’s largest body of anoxic water. In simple terms, that means much of its deeper water contains little or no oxygen. Below roughly 100m the sea becomes increasingly hostile to most familiar marine life. This “dead zone”, which makes up more than 80 per cent of its waters, is inhabited only by a few anaerobic bacteria.
The Black Sea is almost landlocked, joined to the Mediterranean only by narrow, shallow waterways. As a consequence the circulation of oxygen-rich waters is limited, resulting in “vertical stratification”. Fresh water from rivers forms a lighter surface layer, while denser, saltier water from the Mediterranean sits beneath it. Over time, the deep water has become anoxic and is instead rich in hydrogen sulfide (a chemical produced by anaerobic bacteria).
For wildlife, that deep zone is a barrier. For archaeologists, it is a gift. Without oxygen, the bacteria and wood-boring creatures that usually destroy shipwrecks struggle to do their work, helping to preserve ancient vessels on the seabed.
What lives in the Black Sea?
The deep may be strange and largely lifeless, but the upper waters are far from empty. The Black Sea supports plankton, jellyfish, molluscs, crustaceans, seaweeds, fish and marine mammals. Its best-known residents include bottlenose dolphins, common dolphins and harbour porpoises. WWF notes that the Black Sea is home to about 180 fish species.
- It’s snowing underwater? How in the great depths of the ocean there's a constant snowstorm
- It's the lowest place on Earth, the size of Seoul, 10 times saltier than the ocean – AND it's dying. One day soon it might not be here...
Fish found here include anchovy, sprat, turbot, mullet, gobies and sturgeons. The sea also supports globally important wetlands and river deltas around its basin, including the Danube Delta, one of Europe’s great wildlife hotspots.
Is the Black Sea dangerous?
The Black Sea is not packed with man-eating monsters, but it does contain animals that deserve respect. A study of venomous fishes on Black Sea coasts identified four important species: common stingray, greater weever, scorpion fish and stargazer. Their spines can deliver painful venom, particularly if stepped on or handled carelessly.
For most swimmers, though, the bigger dangers are likely to be the ordinary hazards of the sea: weather, currents, deep water and human activity.
What threats does it face?
The Black Sea has had a rough time. Pollution is a particularly pressing issue because the sea is almost landlocked and anything that enters from rivers, cities, farms, industry and shipping does not simply flush away into the open ocean.
Overfishing is also a major concern. Some commercially valuable species, including turbot, anchovy, whiting and horse mackerel, have been heavily exploited, while the Black Sea’s sturgeons have suffered from a mixture of overfishing, illegal fishing and damage to the rivers where they spawn.
Then there are invaders. Perhaps the most infamous is the comb jelly Mnemiopsis leidyi, which arrived from the western Atlantic and spread rapidly in the 1980s. By feeding on zooplankton, fish eggs and larvae, it helped push an already stressed ecosystem into crisis. Other non-native species have followed, often helped by shipping and ballast water.
The Black Sea’s marine mammals face their own pressures, namely underwater noise and marine traffic, which is especially damaging to the resident bottlenose dolphins, common dolphins and harbour porpoises.
There is still plenty to protect. The Black Sea more than lives up to its mysterious name. It is a wildlife haven at the surface and, thanks to its oxygen-starved lower layers, an extraordinary underwater museum.
