It's almost the same size as Switzerland, home to a 6.5 foot giant, a tiny, very deadly, octopus and is slowly turning the wrong colour...

One of four seas named after colours, the Yellow Sea is a stretch of water about 40,000km2 between mainland China and the Korean peninsula that receives so much mineral-rich soil from the rivers that flow into it, the water has a golden-yellow colour.

The Yellow River (Huang He) and the Yangtze River are the source of the 1.6 billion tons a year of yellow-ish sediments that arrive in the Yellow Sea. But over the last few decades the colour of the Yellow Sea has been changing.

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What is the Yellow Sea?

The Yellow Sea is one of the world's largest areas of continental shelf - a sloping perimeter of continent that formed after the last glacial period around 10,000 years ago. As a result it is not that deep - the Yellow Sea's depths are, on average, only between 60 and 80 metres.

This makes the Yellow Sea and its mudflats a brilliant home to migratory birds and shorebirds - and it is situated in the East Asian-Australasian Flyway. It is estimated that 50 million migratory birds pass by the Yellow Sea each year.

What lives in the Yellow Sea?

The Yellow Sea is home to a vast number of animals, from Pacific herring, to Japanese mackerel, cod, marine turtles, porpoises and dugongs. Great white sharks prowl the waters of the Yellow Sea, looking for spotted seals - the sea's only resident species of seal.

However, in recent years the fishing waters have been much depleted and instead huge floating giants have taken over. Great white sharks aren't the only enormous animal you'll find in the Yellow Sea. The area is home to Nomura's Jellyfish which can span 6.5 feet and weighs up to 440 pounds.

Among the largest jellyfish species in the world, Nomura's Jellyfish begins life as a tiny grain of rice and can grow to the size of an adult human in a year. Its venom can be highly painful - causing itching, swelling rash and - and in severe cases it can lead to death.

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But it's the sheer number of the giants that causes the most amount of damage - jellyfish blooms happen when there is a rapid explosion in population and when this happens it can wreak havoc on fishing equipment.

Some animals that live there are tiny and deadly

Also to be found in the Yellow Sea is blue-ringed octopus, one of the most deadly octopuses in the world. It is tiny - measuring around 12 to 20 cm - and although generally quite docile, it carries enough venom to paralyze and kill 26 adult humans. These remarkable animals can quickly change colour and when they are threatened their blue rings flash. Their bites are small and often painless, which means victims often don't realise they've been bitten until it is too late.

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Algae blooms in the Yellow Sea

The Yellow Sea is also home to many different types of seaweed including Ulva prolifera - a blue-green algae - which helps to turn the water a slightly different hew in summer. While the algae is not toxic to humans or animals, large mats of it can still deplete water of oxygen which is not good for fish, and in 2021 the Yellow Sea became bright green during the largest blue-green algae bloom on record.

Scientists have been monitoring the size of blue-green algae blooms which generally fluctuate year to year, and since 2012 the blooms have been increasing in size. But that's not the only reason the Yellow Sea's colour has been changing.

Although it was once rich in flora, fauna and megafauna, the populations have decreased in recent years and the Yellow Sea is now considered one of the most degraded marine areas on earth, with natural habitats lost due to land reclamation. About 40 per cent of the tidal flats have been reclaimed over the last 100 years.

Now the waters of the Yellow Sea are more often brown than yellow, because of decades of industrial pollution, agricultural runoff and domestic sewerage, which is one of the reasons why blue-green algae thrives. By the 1990s it was thought that fish and invertebrate populations had declined by 40 per cent.

Since 1997 there have been several partnerships set up between countries to try to save the Yellow Sea and find solutions to combat marine pollution, reduce overfishing and to restore habitats.