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Here on land, steep slopes can  give way to enormous landslides. But even the biggest ones are nothing compared to the landslides that happen under the ocean. Underwater landslides can be the size of entire cities and trigger gigantic tsunamis.

But weirdly enough, it doesn’t  take a dramatic event like an earthquake or a volcano to set one off. The things causing many of  these enormous slides might actually be so tiny you’d  need a microscope to see them. [♪ INTRO] Most massive landslides happen at  the edges of continental shelves. That’s where the shallow ocean floor surrounding the continents drops off into deep ocean basins.

These transition areas are  called continental slopes. But it’s not like the sharp drop-off  into the deep end of a pool. These slopes are mostly gradual, often not even half as steep  as a standard wheelchair ramp.

On land, that kind of gradient  normally wouldn’t send land sliding. But all over the world, massive  chunks of the ocean floor hurtle down these continental slopes in so-called mega-slides. As catastrophic as that might sound, events like this are actually really  important for ocean ecosystems.

The continental shelves are home to  the vast majority of the oceans’ life, so they’re full of organic matter. Mega-slides transfer this nutrient-rich  sediment from the coastal areas into the deep ocean, which is more barren. Most of the time, we never even  notice as these big chunks of earth rearrange themselves deep underwater.

But sometimes, they can affect people. Over 8,000 years ago, an epic  mega-slide in the North Sea sent a wave up to 20 meters  high crashing to shore. We don’t know the death toll, but  some scientists think this wave dealt the final blow to an ancient  human settlement known as Doggerland.

Thankfully, this kind of thing doesn’t  happen a lot, but it’s not so rare either. In 1999, an underwater landslide  created a tsunami off the coast of Papua New Guinea that killed over 2,000 people. These days, mega-slides can also  damage critical infrastructure.

For instance, they’ve been known to  break cables running along the seafloor, which we rely on for the internet  and other global communications. And they could damage other  structures we build offshore, like oil platforms and wind turbines. So, when it comes to mega-slides, it’s  not just “out of sight, out of mind.” There’s a lot at stake here.

Which  gives scientists plenty of reasons for wanting to know what causes  mega-slides in the first place. Unfortunately, finding an answer is complicated because the seafloor is a super complex place. There are a bunch of things happening at once.

You’ve got currents, sediments, and  tectonic plates all moving around. Plus, it’s not easy to explore the ocean floor. We can’t just send scientists to scope  things out the way they do on land.

And without knowing when and where  the next mega-slide will happen, it’s hard to detect these things in real time. So far, scientists’ best bet  has been to piece together clues from mega-slides that have happened in the past. One important clue came from  the fact that these slides happen on shallow slopes.

For that to be possible,  researchers were pretty sure there had to be a weak layer that was  giving way somewhere in the slope. And once that gave way, everything  on top of it was basically just sledding downhill on that weak layer. The question was just: What was that layer?

Some thought it might be a  layer of rock or sediment that was prone to breakage or collapse. Others thought it might be grains  that were saturated with water. In a 2018 study, one team  considered another possibility: What if it was a layer of tiny dead organisms?

To get to the bottom of this, they  studied the remains of a landslide that happened nearly 150,000 years  ago off the coast of Mauritania. To find the weak layer, they  compared two datasets that gave them a glimpse at the sediments  piled under this part of the ocean floor. One was from a technique  called seismic reflection.

This is a way of figuring  out what rock layers exist under the surface without directly seeing them. Basically, you use a kind of  gun to create seismic waves. Then by measuring how those  waves bend and change speed as they travel through  different layers of sediment, you can figure out what those layers are made of.

The team got this data from the area where a chunk of land had broken off during the slide. Their second dataset came from a sediment core. A sediment core is a long tube-shaped sample that has been drilled out of the  ground, or in this case, the seafloor.

It gives scientists a direct look  at the layers in a certain area. This one was taken from a region near the  landslide that had not been disturbed. With these two datasets in  hand, the authors of the study matched up the layers in each  one so they could compare them.

And sure enough, in the undisturbed core sample, right where the land had split apart in the slide, they found the unstable layer  they’d been looking for. It was just a few meters thick, which is  not thick at all on geological scales. And it was sandwiched between  two stable layers of sediment.

Of all things, this layer was  made of microscopic, dead algae. Or, as scientists actually call it: ooze. This ooze began forming as algae  fell to the seafloor after dying.

The layer of corpses got saturated with water, and as layers of sediment piled  up on top and squished it, that water got squeezed out. Except, the water couldn’t go anywhere. The ooze was topped by a layer  of clay that sealed it off and trapped the squeezed-out water in place.

As sediment kept building up, pressure  built up on the layer of ooze. Eventually, as the trapped water  exerted its own pressure outward, the boundary between the ooze  and the clay became unstable. Once you have this precarious setup,  the structure of the slope can fail, sending all that material above  the weak layer tumbling downhill.

While this study just looked at  one mega-slide from a long time ago, this kind of phenomenon could be  behind many other slides, too. Weak layers of ooze might spread  across thousands of square kilometers, leaving huge chunks of the seafloor  balanced on wobbly slabs of dead algae. And in the future, understanding how  these buildups of microscopic algae lead to monstrous landslides may  help us keep coastal regions safe, which is no small thing.

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