On September 16, 2023, seismologists around the globe picked up an unusual signal: a gentle tremor repeating every 90 seconds. It lasted for nine days, then vanished — only to return a month later, though less intensely. The frequency didn’t match any known earthquake. No volcanic eruption had occurred. There were no storms or nuclear tests. Yet, the entire planet was pulsing.

Now, scientists from the University of Oxford have cracked the case. In a study published in Nature Communications, researchers present the first direct observations of a rare oceanic phenomenon called a seiche. This was a standing wave trapped inside a Greenland fjord, set in motion by two massive, climate-driven landslides that created “mega-tsunamis.” These waves sloshed back and forth for days, subtly tugging at the Earth’s crust — and sending signals that circled the globe.

What caused the Earth to tremble every 90 seconds?

The world is riddled with thousands of seismographs. These instruments form a global network that continuously monitors the Earth’s vibrations, allowing scientists to detect and analyze everything from major earthquakes and volcanic eruptions to distant nuclear tests. Because the physics of seismic waves is well understood, most signals picked up by this network can be quickly traced to their source. So, when a truly strange signal shows up — like the 92-second pulses in 2023 — it stands out immediately, prompting deeper investigation.

Right off the bat, the educated guesses suspected a wave of some sort. But for a year, scientists only had indirect evidence to support this idea. Two studies proposed that the pulsation came from two mega tsunamis triggered in the remote Dickson fjord in East Greenland. The studies went even further, claiming that the tsunamis were caused by two major landslides which occurred due to the warming of an unnamed glacier. This would have created giant waves trapped in the fjord system, forming standing waves (or seiches) that undulated back and forth, causing the mystery signals.

It was a compelling hypothesis, but it lacked the smoking gun. Seismic models and tsunami run-up maps were suggestive, but not conclusive. That changed thanks to a groundbreaking Earth observation satellite.

A team of researchers from Oxford University used SWOT, the Surface Water and Ocean Topography mission, to look for culprits. Launched in December 2022 by NASA and CNES (the French space agency), SWOT carries a sophisticated radar instrument called KaRIn that can scan the Earth’s surface water in wide swaths, rather than in single lines like older satellites.

By analyzing SWOT’s high-resolution “pixel cloud” data, Monahan’s team mapped the water’s surface inside Dickson Fjord in the days after the landslides. They saw clear tilts in the water level — up to two meters high — repeating in opposite directions. These slopes lined up perfectly with what you’d expect from a standing wave sloshing side to side. The timing fit perfectly. Seismologists had their smoking gun.

A freak wave caused by climate change

A short-period seiche persisting for several days without an external driver has not previously been observed, the researchers write. To be certain that nothing else is happening, the researchers ruled out other possible causes like tides or wind-driven currents. They found that tidal patterns in the fjord couldn’t produce the observed two-meter cross-channel slopes, and the winds at the time were too weak or in the wrong direction.

This left only one possible explanation: a massive seiche, triggered by a tsunami, triggered by ice melt. The wave reached a height of about 7.9 meters. That aligns with previous simulations but offers a more confident estimate.

The second event in October produced a smaller wave — roughly 60–75% the size of the first — but was still powerful enough to shake distant seismic stations. This second seiche also showed up clearly in SWOT data just 12 hours after the landslide.

But the curious part is that all this, this giant signal that rippled throughout the Earth multiple times, was essentially caused by climate change. Heating melted the ice, which triggered the landslide, which triggered the tsunami.

“Climate change is giving rise to new, unseen extremes. These extremes are changing the fastest in remote areas, such as the Arctic, where our ability to measure them using physical sensors is limited. This study shows how we can leverage the next generation of satellite earth observation technologies to study these processes,” says Thomas Monahan, from the University of Oxford, who led the study.

As climate change accelerates glacier retreat and increases the risk of landslides and megatsunamis in polar regions, tools like SWOT will become vital to understand what’s happening. Co-author Professor Thomas Adcock, also from the University of Oxford, says new satellites offer a way to study such extreme phenomena where we don’t have other equipment.

“This study is an example of how the next generation of satellite data can resolve phenomena that has remained a mystery in the past. We will be able to get new insights into ocean extremes such as tsunamis, storm surges, and freak waves,” Adcock concludes.

Journal Reference: Observations of the seiche that shook the world, Nature Communications (2025). DOI: 10.1038/s41467-025-59851-7. www.nature.com/articles/s41467-025-59851-7