00:00The snowy coat above you suddenly folds in two,
00:05unveiling the icy slope to its resting point well below.
00:10On its way, everything is crumbled under the enormous mass of snow and debris.
00:15You are witnessing an avalanche, one of the most dreadful natural disasters ever.
00:21And if you were told about another type of avalanche, just as terrifying as that of the mountains,
00:26what makes it particularly strange is that it occurs underwater.
00:31Underwater avalanches are natural phenomena of devastating power,
00:36often far more formidable than terrestrial ones.
00:39They can cause planetary chaos with dramatic consequences.
00:44The most disturbing is that they constantly occur under the surface of the oceans,
00:49invisible, unpredictable and extremely difficult to measure.
00:54These underwater avalanches can reach a magnitude 100 times greater than those of terrestrial avalanches
01:00and gravely threaten global communications,
01:03to the point of potentially depriving the entire planet of internet.
01:07But we will come back to this later.
01:09For now, let's go back 60,000 years in the past.
01:13Here we are off the northwest coast of Africa,
01:16about to discover the magnitude and devastating impact of a titanic underwater avalanche.
01:23It began with a simple slip of underwater terrain in the Agadir Canyon.
01:28Originally, it was only one and a half cubic kilometers of material.
01:33However, this mass was quickly multiplied by more than 100 by crossing the bottom of the Atlantic Ocean.
01:41As the avalanche gained speed and volume,
01:44it carried with it gravel, sand, rocks and mud.
01:48It dug a devastating trench through one of the largest underwater canyons in the world,
01:53eradicating any form of life on its way.
01:56The power of the destructive flow was such that it eroded a segment of 400 kilometers of the canyon,
02:01digging hundreds of meters into its walls and damaging a total area of 4,500 square kilometers,
02:07more than three and a half times the surface of New York.
02:10As for its strength, it projected huge rock blocks along the walls of the canyon,
02:15reaching heights of up to 130 meters.
02:19Before the scientists highlighted this devastating event,
02:23they had largely underestimated the extent of the destruction caused by underwater avalanches.
02:29In addition, these major disasters can occur not only offshore, in the ocean depths,
02:35but also near the estuaries.
02:38Let's go back to January 14, 2020 to examine this more closely.
02:42A current of turbidity, a rapid flow of water descending loaded with sediment,
02:46traveled more than 1,100 kilometers from the estuary of the Congo River to the abysses.
02:51This phenomenon was caused by two factors.
02:53Exceptional floods along the river at the end of December 2019
02:58and spring tides of an unusual magnitude.
03:01This generated a gigantic avalanche of sand and mud,
03:05of which the volume represented a third of all the sediments transported annually by all the rivers on the planet.
03:11This avalanche saw its speed increase,
03:14going from 5 meters per second in the upper part of the Congo Canyon
03:18to 8 meters per second at the end of the canal,
03:21after 1,200 kilometers from the coast.
03:24This journey made it the longest avalanche of sediment ever measured on Earth.
03:28Two days after its launch, the flow reached the abysses.
03:32The avalanche cut two underwater telecommunications cables,
03:36drastically slowing the transfer of Internet data
03:39across West, Central and South Africa.
03:43Before that, scientists considered it impossible to directly measure large-scale underwater avalanches.
03:50However, the flow of sediment observed in the Congo Canyon
03:54allowed to monitor the phenomenon,
03:56offering researchers a better understanding of the link
03:59between large riverbanks and ocean depths.
04:03They also determined that the frequency of underwater avalanches
04:07depended on the location of the observer.
04:10Submarine canyons, located near the mouth of rivers,
04:13can detect several small avalanches each year.
04:16However, systems far from riverbeds, such as the Agadir Canyon,
04:21only experience a titanic avalanche every 10,000 years or so.
04:26You may be wondering about the possible causes of an underwater avalanche.
04:31Earthquakes, typhoons, high tides, river floods
04:35and even volcanic eruptions can be responsible.
04:39In addition, with climate change, these phenomena become more and more frequent and violent.
04:45However, the presence of these triggers does not guarantee that an avalanche will occur
04:50or determine its magnitude.
04:52Let's take an example.
04:53In 1755, a powerful earthquake hit the Portuguese coast,
04:58devastating Lisbon and causing tens of thousands of deaths.
05:02However, this event only caused a small underwater avalanche.
05:07However, in 1929, a major earthquake off Newfoundland, Canada,
05:12triggered the largest underwater avalanche ever documented.
05:17The flow, traveling at 70 km per hour,
05:20carried rocks, sand and mud while breaking 11 underwater cables.
05:26The power of the avalanche also generated a tsunami,
05:29causing the death of 28 people along the coast.
05:33Underwater avalanches are invisible because they occur in depth,
05:37which makes their study extremely complex.
05:40However, their understanding is crucial.
05:42These flows transport sediments, nutrients and even pollutants through the seabed,
05:48playing an essential role in oceanic ecosystems.
05:52A team of researchers from Liverpool then started to solve this mystery.
05:57Over the past four decades,
05:59they have collected more than 300 samples from the ocean floor,
06:03associating them to seismic and bathymetric data.
06:06In other words, to the cartography of the seabed.
06:09They were able to reconstruct a huge underwater avalanche,
06:12a first at this scale.
06:15What makes the event even more fascinating
06:17is that it started relatively modestly
06:19before transforming into a gigantic avalanche,
06:22reaching more than 200 meters in height.
06:25To imagine the magnitude,
06:26think of an avalanche the size of a skyscraper in Liverpool, London,
06:30digging a trench 30 meters deep and 15 kilometers wide.
06:35Finally, it spread over a surface higher than that of the United Kingdom,
06:40covering everything with a thick layer of sand and mud.
06:43It's like burying an entire country.
06:46These discoveries are truly crucial,
06:49because they reveal that a low-magnitude undersea landslide
06:53can evolve into a gigantic and highly destructive phenomenon.
06:57But the most worrying are the underwater internet cables,
07:01essential to global communication.
07:04There are more than 550 active cables on the seabed,
07:07totaling an incredible length of 1,400,000 kilometers,
07:11which is enough to surround the Earth 35 times.
07:15When such an avalanche damages underwater cables,
07:18the consequences are often catastrophic and costly.
07:22In 2006, a typhoon in Taiwan caused underwater avalanches
07:26that cut several cables linking Southeast Asia to the rest of the world.
07:31This led to a loss of 90% of internet traffic
07:35between China and the United States at the peak of the event.
07:38Taiwan, on the other hand,
07:40experienced traffic interruptions going from 74% to 100% to neighboring islands.
07:45You now understand why it is essential to know more about this natural phenomenon.
07:50Fortunately, researchers at the University of Tulane
07:54have found a way to analyze it in three dimensions.
07:57So far, studies have only considered undersea avalanches in two dimensions,
08:02which did not fully capture their behavior.
08:07Researchers have focused their efforts on the interaction of undersea flows
08:11with mini-pools, natural formations on the ocean floor
08:15resembling shallow bowls.
08:17Despite their name, these pools can be vast,
08:20reaching up to 10 square kilometers,
08:23which is about twice the area of Central Park in New York.
08:27When currents of turbidity enter these mini-pools,
08:31they do not simply move.
08:33On the contrary, they hit the opposite wall
08:36and return in the form of whirlpools,
08:38similar to tides in a watercourse.
08:41These whirling movements redistribute the sediments
08:45before they settle.
08:47The size of the pool and the speed of the current
08:50strongly influence this process.
08:53If the pool is large and the current is slow,
08:56it gradually fills up with sediments.
08:58But if the current accelerates or if the pool is reduced,
09:01the flow overflows,
09:03spreading beyond its limits.
09:06To further their research,
09:08scientists have created a replica of a mini-pool in a swimming pool
09:12and tested various flow speeds
09:15to simulate currents of turbidity.
09:17They even designed specific sediment mixtures
09:20to observe the behavior of particles of various sizes.
09:24Finally, they succeeded in reproducing oceanic conditions,
09:28but on a controllable scale and adapted to their experiments.
09:32Fascinating, isn't it?
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