The river cascades through steep-sided gorges and churns around isolated towers of rock, before winding across a vast plain beyond. It is a torrent to rival the mighty Colorado River that carved out the Grand Canyon.

Yet this dramatic natural wonder has never appeared in any tourist photographs, nor does it feature on any maps. The reason? It lies two miles (3.2km) beneath the surface of the Pacific Ocean.

Stretching away from the coast of California, the Monterey Canyon has been shaped over millions of years by this bizarre “undersea river”. Beyond the mouth of the canyon, the flow has cut a valley into the sea floor that extends for nearly 200 miles before spilling onto the abyssal plain of the deep ocean.

Similar channels can be found etched into seabeds all over the world. They have been found off the coasts of Greenland, the Amazon, the Congo and Bengal, to name a few. The largest are several miles wide and run for thousands of miles out into the ocean depths, where they provide vital sustenance to the creatures living there. But these undersea rivers are among the least understood phenomena on our planet.

In many ways, undersea rivers are similar to the rivers we see on land. They have banks on either side. Smaller rivers called “tributaries” feed into larger ones. The rivers carve valleys into the sea floor. They follow meandering paths and can even change course, resulting in abandoned sections similar to oxbow lakes. Ultimately, they spill out onto the abyssal plain in the ocean depths in similar ways to a river estuary.

“If you drained all the water away, it would look exactly like a river system with bends and meanders, except there are no trees along the banks,” says Dan Parsons, a sedimentologist at the University of Hull, UK, who travels the world to study undersea rivers .

These submarine channels were almost completely unknown until the 1980s, when sonar mapping of the seafloor began to reveal them. Many extend out into the ocean from the mouths of major rivers like the Amazon and the Congo, following tortuous routes across the thick sediment on the seafloor. At the time, scientists compared them to mature river systems on land, such as the lower reaches of the Mississippi River.

The underwater canyons cut into the continental shelf can be compared to the headwaters of a river system. From there, a river will spill into large meandering channels that extend out on the floor of the continental slope and the continental rise.

There, the channels tend to be bounded by huge levees that have built up over time. Some of these levees stand hundreds of feet above the sea floor. “These submarine channel systems are some of the biggest in the Solar System,” says Parsons.

However, it was only in the late 1990s that it became clear how these channels were created.

Rather than fresh water flowing along them, these underwater rivers are slurries of silt and sand that cascade along channels on the seabed

Scientists drilled into the sediments in the channels, and the sediment cores indicated they had formed through repeated deposits of sediment that appeared to spill down the channels.

Rather than flows of fresh (or at least salty) water, undersea rivers are slurries of silt and sand that cascade along channels on the seabed. Each particle tumbles through the water under its own weight. A new river starts on the continental shelf like an avalanche in the mountains, picking up speed and momentum as it moves until it flows like a liquid. Once started, an undersea river can flow for weeks and even months at a time, moving the same amount of sediment in one go that all the world’s land-based rivers transport in an entire year.

“The flows that come down them are more like snow avalanches or volcanic pyroclastic flows,” says Parsons.

However, studying these processes in the deep sea has proven difficult.

“When you compare them to what we know about rivers on land, we have practically no measurements of these flows under the sea,” says Parsons.

Part of the reason for this is the difficulty in studying an environment at that depth. Many of the channels are found more than a mile (2km) down and can flow to depths of 2.5 miles (4km). To reach these inky depths requires specialised remote controlled deep-sea submarines.

Worse, the rivers only flow some of the time. After a flow has passed, the channel may be inactive for weeks, months or even years. It can cost over £25,000 a day to use a research vessel that can launch remotely operated vehicles (ROVs) to explore the deep ocean, so it is hardly surprising few scientists have been able to study these undersea flows.

“People have just not had the capacity to go and look before,” says Jeff Peakall, a sedimentologist at the University of Leeds, UK. “In fact, we have better resolution of the far side of the Moon than we do under our oceans. We know remarkably little about these underwater rivers.”

Instead, for many years researchers had to rely on laboratory simulations, mixing seawater with building plaster or mud in large tanks to create turbidity currents. Footage of these experiments reveals that the currents are similar to avalanches or pyroclastic flows, as the sediment billows and surges along the bottom of the tanks.

Now a small band of intrepid researchers are beginning to explore these deep-sea channels and learn more about how they work and what lives around them. “We are now at the stage where the technology is letting us measure the flows in the real world at full scale,” says Parsons. “That has not been possible until relatively recently.”

In a study published in 2010, Peakall and Parsons sent a robotic submarine down to a deep undersea channel that runs across the bottom of the Black Sea

“In Monterey, we are repeatedly mapping the canyon to see how it changes over time,” says Parsons. His team is using autonomous underwater vehicles to show how the flows are changing the seafloor.

Parsons is also conducting similar research off the coast of British Columbia. There, meltwater from glaciers and snow in the mountains delivers sand and mud to a delta at the top of a fjord known as Bute Inlet, which will then collapse and flow down the submarine channel in a powerful “turbidity current” – the name given to these cascades of sediment laden water.

In a study published in 2010, Peakall and Parsons sent a robotic submarine down to a deep undersea channel that runs across the bottom of the Black Sea. Here, they found another type of current is carving the river channel, this time a flow of salty water, which originally comes from the Mediterranean, spills into the Black Sea through the narrow Bosphorus strait and then into the channel.

Since the Mediterranean water is saltier, and so denser, than the Black Sea, it remains separate, flowing at a speed of around 4mph (6.4km/h). Every second, around 22,000 cubic metres of water passes through the channel.

“We were trying to map the water coming out of the Bosphorus strait, to understand how flows move through undersea rivers,” says Parsons. “It was a warm-up for looking at the really big systems driven by mud and sand in the offshore submarine channels.”

Only in the last few years have researchers witnessed an undersea mud river in the real world. In 2013, Charlie Paull of the Monterey Bay Aquarium Research Institute and his colleagues were using an ROV to explore a relatively small underwater canyon, just a few miles from the Californian coast. The ROV was tethered to a ship on the surface, around 1,500ft (400m) above.

Without warning, a turbidity current came roaring down the canyon and the ROV was caught in it. One of the ROV pilots described the experience as being like “flying an ROV in a tornado”. The five-ton ROV was lifted off the seafloor and pushed sideways.

The sheer power of these enormous flows of sediment can make studying them a challenge

It beamed back video, which revealed a dense layer of muddy water surging and billowing across the canyon valley. Clumps of kelp, torn from the seabed further upstream, could be seen rushing past. But before they could learn more, the team had to pull the ROV out of the flow, for fear it would be torn free of its tether.

A 2014 analysis by Esther Sumner of the University of Southampton showed that the sediment flow, which was 295ft (90m) thick from top to bottom, had travelled down the canyon at around 3.8mph (6.1km/h).

Yet this was a relatively small flow.

Oceanographers at the Monterey Bay Aquarium Research Institute are now leading the development of new technologies to study bigger rivers. They have developed acoustic “speed cameras”, which can measure the speed of the flows tumbling down the Monterey Canyon and into the valley beyond.

They have made “smart boulders”: beach-ball-sized instrument arrays, also known as benthic event detectors or Beds. These can sit on the floor of the channel. When a sediment river cascades down, they are picked up and carried along. They send back information about how they roll, glide and lift from the sea floor.

Yet the sheer power of these enormous flows of sediment can make studying them a challenge. In January 2016, Paull and his team lost a fixed monitoring device, along with the one-tonne tripod it was mounted on, when a powerful sediment flow swept down the Monterey Canyon at 12mph (19.3km/h). They eventually found it, after following the pings from its beacon - three miles from its original position, almost completely buried in mud. When they managed to pull it out, they found steel plates on the frame had been bent out of shape and ground down to a knife-edge. A float on top of the tripod, made of carbon fibre and titanium, had also been badly eroded. Ten months later, they lost a second tripod in a similar manner, while another event saw an entire mooring dragged four miles (7.1km) out of position.

“It is sobering to think those sort of events are going on under the seabed,” says Paull. “When I look out of my window at the ocean, there is no sign of these powerful events taking place, but on the sea floor they are powerful enough to drag entire boulders with them.”

Faced with this kind of destructive force, it is hard to imagine much life surviving along the length of these undersea channels. Yet some species, at least, seem to thrive.

The channel beds teem with snails, clams, crustaceans, urchins, sea cucumbers and worms

“These sediment flows have a major impact on canyon biodiversity,” says Craig McClain of the Louisiana University Marine Consortium, who has been working with the team at Monterey Canyon. “For some types of species, this disturbance causes a boom allowing their numbers to grow quickly, while for others their numbers plummet. It depends on whether a species is a ‘weedy’ species with fast growth and reproduction or not.”

With Jim Barry at the Monterey Bay Aquarium Research Institute, McClain has shown that the channel beds teem with snails, clams, crustaceans, urchins, sea cucumbers and worms.

What’s more, beyond the safety of the canyon, the nutrients and oxygen carried by the flows seems to help life survive on the comparative desert of the ocean’s abyssal plain.

To find out what is going on, scientists have looked at sandstone that was formed from sediment flows under prehistoric oceans. Telltale holes in the rocks suggest small worms once burrowed through the sediment.

“What appears to happen is the flows not only bring oxygen and nutrients down to the deep ocean, but they also carry life with them too,” says Peakall. “These worms are swept down from shallower depths and live in the sediment when it settles, until they run out of oxygen.”

A 2016 experiment by Sumner suggests that polychaete worms – brightly-coloured marine creatures, covered in bristles like a pipe cleaner – may be able to survive such a journey intact.

The organisms living in the sediment may also play an important role in the way these undersea rivers flow in the first place.

A 2015 study by Jaco Baas of the University of Bangor, UK, and his colleagues showed that microorganisms living in the mud help to bind it together, allowing sediment to pile up – until it fails catastrophically. This helps explain why undersea rivers only flow periodically.

“The biggest flows are probably triggered by a failure in the sediment building up on the continental shelf,” says Peter Talling, a geologist at the University of Durham, UK. “Flooding or waves during storms can cause an underwater avalanche. Fresh water from rivers filled with sediment can also be denser than sea water, and so plunge to the bottom of the ocean.”

It now seems that something strange happens to the flows as they go downstream. Studies of undersea rivers off the coast of the Congo show that they stretch as they go downstream. This means an event that lasts an hour at the top can go on for days or weeks at the bottom of the channel.

These are incredibly powerful and destructive flows – Dan Parsons, University of Hull

One of the most active undersea sediment rivers can be found in the Nazaré Canyon off the coast of Portugal. The river runs down a narrow channel inside the five-mile-wide (8km) canyon, before flowing across the abyssal plain nearly 2.5 miles (4km) beneath the surface, where it is contained with large levees.

Around four times a year, small flows spill down the Nazaré Canyon for a few kilometres at a time before running out of steam. But the canyon is sometimes hit by more violent events.

“At the extreme end are what we call ‘canyon flushing’ turbidity currents,” says Josh Allin of Southampton University in the UK, who has been studying the Nazaré canyon. “These are much more violent and are capable of eroding very large volumes of sediment – tens of cubic kilometres – from the canyon and transporting it out onto the deep ocean floor. They appear to occur on hundred- to thousand-year timescales, but they have never been directly observed and we know very little about their characteristics.”

While canyon flushing turbidity currents are rare, it could still be important to understand them. For starters, they help to lock away huge volumes of carbon in the sediment at the bottom of the ocean, slightly slowing the rise in greenhouse gases that is causing climate change. But they can also have more immediate effects on our lives.

In 1929, 23 underwater telegraph cables were cut close to Newfoundland. It was suggested later that an offshore earthquake had struck the nearby Grand Banks, dislodging a bank of sediment, which then roared down a channel in the continental shelf and out onto the abyssal plain.

Today, nearly all of the world’s internet and banking transactions are conducted over underwater cables, so if a lot of these cables were cut it would cause major problems. Many of the cables connecting the US to Europe cross the path of this same underwater river channel that runs south from Newfoundland. Scientists estimate the flow that came down that channel in 1929 reached speeds of 57mph (93km/h) and carried debris more than 683 miles (1,100km) across the sea floor.

“If we saw a repeat of that now, it could be disastrous,” says Talling. His research is partly funded by the International Cable Protection Committee, which looks after underwater infrastructure.

“These are incredibly powerful and destructive flows,” says Parsons. “It is important we understand how they work.”

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