On September 16th, 2023, earthquake detectors around the world started  picking up a strange vibration.

It was a deep rumble… a seismic wave, but one that didn’t look anything  like a regular earthquake. The noise was made up of just one frequency, like the sound that comes out of a tuning fork.

And it kept ringing, for nine full days. Earthquake scientists had no  clue what they were looking at. So they spent nearly a year tracking down  the source of this planet-wide shakeup.

And what they found is something  we’ve never seen before… but something that’s bound  to shake up our planet again. [♪ INTRO] After this mysterious signal showed  up on detectors around the world, dozens of scientists from 15 different  countries teamed up to track down the source… or what they called  “the unidentified seismic object.” One key clue was when it first  hit each monitoring station. Given how long it took for the  vibration to reach each one, scientists could estimate how far  away that station was from the source. They used that data to trace the event  back to somewhere in East Greenland.

And that’s where they found their next clue: On the same day the mystery signal started, sea-level gauges in East Greenland  recorded an absolutely massive tsunami. It was around four times  taller than Niagara Falls, making it the tallest recorded  wave on Earth since 1980. So it seemed pretty likely that this  massive wave had something to do with the mystery signal… but before  scientists could figure out what, they had a new question to answer: What had caused this monstrosity of a tsunami?

To solve this puzzle, the team of  scientists turned to satellite images and seismic data to piece together  the events leading up to the tsunami. And what they found was, on the day  that the signal began, a colossal chunk of land broke off a mountain, more  than a kilometer above the water. On its way down, it shattered a glacier,  sending 25 million cubic meters of rock and ice (enough to fill  10,000 Olympic swimming pools) crashing into a nearby fjord.

And this wasn’t just any rockslide. It was really a perfect storm: It had all  that extra debris from the shattered glacier, plus an icy slope acting  like a natural slip-n-slide. All that energy got funneled into a narrow gully, so by the time the rocks and ice  splashed down into the fjord, they packed a massive punch and  kicked up an absolutely enormous wave.

So, scientists finally had a picture of  what happened the day the signal began. But that still didn’t answer the  question of what caused the noise. Because the landslide was over in minutes.

How could it explain a signal  that went on for /nine days/? There was only so much scientists  could figure out from the data they happened to have on this event. So to piece together the finer  details, they created a simulation that would let them model how  the whole disaster unraveled.

But to do it, they needed funding. And so do we, so let’s tell you about ours. Thanks for watching this SciShow video!

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Simulating the details of  the event was a great call, and it turned up something really surprising: Instead of crashing and  dissipating like a typical tsunami, their wave got trapped between  the walls of the fjord. And for days, it sloshed back and  forth, just like a wave between the walls of a bathtub… if that  wave were about sixty stories tall. Now, a wave like this isn’t unheard of.

It’s called a seiche, and you  get one now and then in bodies that are partly or fully closed  off, like lakes and harbors. They can happen anytime some force, like a strong wind or a tsunami, pushes  a bunch of water up against a shoreline. When the thing doing the pushing lets up, all that water slides back and  bounces off the opposite shore.

And this kind of wave can  keep going for hours or days, until it gradually loses its energy and dies off. The seiche in their simulations  bounced off opposite shorelines every 87 seconds… and that’s  when scientists realized they just might have cracked the case. Because one cycle every 87  seconds was awfully close to the period of the mystery signal they’d  detected the day of the landslide.

To check if their simulation was  really capturing what had happened in the real world, scientists looked  at a few other lines of evidence, and everything seemed to line up. The sloshing wave moved in the same direction as the waves detected at the monitoring stations. Small variations in the real signal lined  up with the tidal cycles in the fjord.

And the simulated seiche petered out at  about the same rate as the real signal. So there was their answer:  As best anyone can tell, a tsunami trapped in a fjord shook  our whole planet for nine days. Nearly a full year after the signal was  first detected, the case was finally closed.

But it wasn’t all happily ever after  once the scientists got their answer. Because they realized that this whole  thing wasn’t just a random catastrophe. The seiche that shook up our  planet was like the final step in a Rube Goldberg machine  that got going decades ago.

It all started with climate change and  some unusual configurations of geology. See, this spot has a whole mountain  leaning on a glacier that had lost tens of meters of ice over the years. Finally it got so thin that it could  no longer hold up the mountain.

That’s when a massive chunk collapsed, kicking off the tsunami and then the seiche. Which means that ultimately, this planet-wide seismic signal was triggered by climate change. Scientists had never detected something  like this before in Greenland, but as more glaciers melt,  more landslides are inevitable… and possibly more global shakeups.

So, this mystery signal turned out  to be more than a geological puzzle. It was a signal that we’re living in a  world that’s changing in unpredictable ways. As we get better at cracking the cryptic  messages our planet is sending us, it’s also up to researchers, as well as all  the rest of us, to listen to what it’s saying. [♪ OUTRO]