mmense rivers that dwarf the Thames have been found at the bottom of the ocean by British scientists.
Like those on land, the submarine waterways carve out channels, tributaries, flood plains, rapids and even waterfalls.
One river, discovered underneath the Black Sea, is up to 115ft deep in places and more than half a mile wide.
The underwater giant: The river, shown with a 3D scan using false colour, flows along the bed of the Black Sea
If found on land, scientists estimate the so-far unnamed waterway would be the world's sixth largest river in terms of the amount of water flowing through it.
The flow – carrying highly salty water and sediment - is 350 times greater than the Thames, according to a Leeds University team who used a robotic submarine to scan the seabed near Turkey.
The undersea river - the only active one to have been found so far - stems from salty water spilling through the Bosphorus Strait from the Mediterranean into the Black Sea, where the water has a lower salt content.
This causes the dense water from the Mediterranean to flow like a river along the sea bed, carving a channel and deep banks.
The discovery could help explain how life manages to survive in the deep ocean far out to sea away from the nutrient rich waters that are found close to land, as the rivers carry sediment and nutrients with them.
Dr Dan Parsons, who led a team from the university's school of earth and environment, told the Sunday Telegraph: ‘The water in the channels is denser than the surrounding seawater because it has higher salinity and is carrying so much sediment.
Watershed discovery: The river - with 350 times the flow of the Thames - has channels, rapids and waterfalls
‘It flows down the sea shelf and out into the abyssal plain much like a river on land.
The abyssal plains of our oceans are like the deserts of the marine world, but these channels can deliver nutrients and ingredients needed for life out over these deserts.
‘This means they could be vitally important, like arteries providing life to the deep ocean.
‘The key difference we found from terrestrial rivers was that as the flow goes round the bend, the water spirals in the opposite way to rivers on land.’
Scientists have long suspected seabed rivers can form, after sonar scans revealed meandering channels in many of the world's oceans.
But none have been found before to be have currents of water flowing through them.
Find: The seabed river flows from Turkey's Bosporus
Among the largest of these channels is off the coast of Brazil where the Amazon enters the Atlantic Ocean.
Most are believed to have formed when sea levels were much lower and the channels have been found to be up to 2,500 miles long and be several miles wide.
The channel in the Black Sea, although much smaller, is the only one to be found still flowing and proves that these mysterious channels are formed by underwater rivers.
Unlike ocean trenches, which are geological formations that form at the deepest parts of the ocean due to movements of the tectonic plates, the undersea river channels meander like rivers on land and form banks in the same way by eroding the silt from the bottom of the channel and building it up at the edges.
Dr Parsons found that the Black Sea river is flowing at around four miles per hour with 22,000 cubic metres of water passing through the channel every second – 10 times greater than Europe's biggest river, the Rhine.
The Black Sea river flows only for around 37 miles until it reaches the edge of the sea shelf and dissipates into the deep sea.
Dr Parsons said data from the research will also be important for oil companies looking to drill in areas where these rivers exist.
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