For a while now, the North Sea off the coast of Germany has been the site of a strange phenomenon.

Researchers have noticed that the ocean floor here is strangely pitted, home to thousands, maybe millions, of divots. They can be as shallow as a footprint or as deep as Lake Michigan.

As narrow as a loveseat or as wide as a football field. Often, they’re found in clusters. Scientists in multiple fields have been studying the pits for years, and no one could figure out what was making them.

But new research may have  finally identified the culprit. And if they’re right, it may shed a whole new light on one of the ocean’s big mysteries. [Intro music] Before we get to the North Sea, let's take a trip across the ocean, to Nova Scotia. In the late 1960s, researchers looking at the seafloor off the coast of Canada noticed that it was covered in little pockmarks.

After some investigation, they determined the pockmarks were caused by methane. Our planet has huge  underground pockets of methane. In some places, that methane forces its way up through the seafloor.

Wherever it surfaces, it leaves behind a scar because it disperses the sediment around the hole. Methane isn’t the only thing that can cause these pockmarks - any fluid venting could do it. Closer to the coastline, these marks often form when groundwater bursts out from underground.

But in the deep ocean, it’s mostly methane. And we find those pits all over the world now. So when researchers began to find divots at the bottom  of the North Sea near Germany, methane seemed the likely cause.

That area is a major center  of oil and gas exploration. So finding pockmarks that resulted from hydrocarbons bubbling up made total sense. Except, the more they looked at them, the less things seemed to add up.

They found the pits all over the North Sea, including in a place just  west of the island of Sylt. The pits there mysteriously tended to appear and then get filled in, which researchers believed was  because of storms in the area. Their theory was that methane would bubble up episodically, and then the storms would bring sediments down from the island to fill the pits.

The problem was, when they modeled just how much methane would be needed to regularly create these pits, they found that it was something like 5 kilotons of methane per burst. Which seemed really unlikely  based on their estimates for how much methane could be down there. That left two possibilities.

One, there was way more methane below the North Sea than anyone ever thought, or two, something else was making those divots. This ended up causing some debate between researchers, and that exposed other discrepancies. Some researchers looked at the seafloor and determined that the sediment of the North Sea is way more porous than normal for methane pits.

Finally, a research group out of Kiel University took a ship out and used an echo sounder to examine the pits. That’s a form of sonar that scans the seafloor and is often used to detect gasses below it. They kept at it and measured for thousands of kilometers, but they couldn’t find a trace of methane.

What they did find was that the pits had the wrong shape. Methane pits should be conical, but further examination revealed that the North Sea pits were actually flat circles. This isn’t what methane would produce after bursting through the seafloor.

It looked more like what foraging animals might leave behind after digging for food. But for an animal to cause  the kind of widespread pitting they observed in the North Sea, it would have to be really big. That sounds obvious, but the majority of marine life is actually tiny.

Megafauna, the big animals you can actually see, are pretty rare in comparison  to the size of the ocean. They would also have to be plentiful, because again, there are loads of these pits. So this would have to be their natural habitat.

And these large animals would need a motive. They’d need to be looking for something underground. That led scientists to wonder… could it be porpoises?

Porpoises do dig into the sea floor. They poke at it with their  noses to find sand eels. But this behavior has never been witnessed in the North Sea before, and the scale of seafloor excavation was so massive that it seemed hard to miss.

To find out for sure, geology researchers teamed up  with some marine biologists. They found that the habitats of both porpoises and sand eels did overlap with their study area. And when they went out with their echo sounder to look at the sea floor of  other areas they expected to have both porpoises and sand eels, they found more shallow pits.

Unfortunately, porpoises are camera shy, and the waters of the North Sea are murky. So the researchers were never able to get pictures of porpoises digging for eels. I’m sorry, we would have put  it on screen if we had it, but the pictures literally don’t exist.

But it does now look like the North Sea pits are more likely to be caused by the animals than by methane. What’s surprising about this is that, if true, it’s the first time we’ve  observed marine megafauna influencing the shape of the seafloor in such a big way. The porpoises are doing something called bioturbation, a phenomenon where animals turn over the seafloor, letting oxygen and nutrients mix into the sediment and helping other creatures in the seafloor grow and find food.

But researchers often think of bioturbation as being done by small animals, not marine megafauna. The ocean is vast, so we don’t often think of bigger creatures like porpoises being numerous enough to affect it. It would help us straighten out an important mix-up, though.

Methane seeps are often considered signs of tectonic activity, so when we see them, we start worrying about earthquakes in the region. If these pits aren’t methane seeps, it may mean that we’re worrying about more regions than we have to. Being able to tell the difference between pits made by marine life and those made by methane can help us monitor tectonic activity better, helping with earthquake studies.

So seismologists are among the many who are hoping these pits are, in fact, made on porpoise. Marine animals are just incredible. They're why so many young people  want to be marine biologists.

And if you know a young person who has been expressing that kind of enthusiasm for all things marine, you can fuel that joy by sending them to our SciShow kids channel, where we have tons of videos featuring awesome marine life, like a compilation of the biggest  stuff living in our world. Spoiler alert: One of those animals lives underwater Whether you're on land or at sea, six years old or 96 years old, we're thankful that you watch SciShow. [ OUTRO ]