CNN
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It started with a melting glacier that triggered a massive landslide that unleashed a 200-meter-high mega-tsunami in Greenland last September. Then something inexplicable happened: a mysterious vibration that shook the planet for nine days.

Over the past year, dozens of scientists around the world have tried to figure out what that signal was.

Now they have an answer, according to a new study in the journal Science, and it provides another warning that the Arctic is entering “uncharted waters” as humans continue to drive global temperatures higher.

Some seismologists thought their instruments were broken when they began recording vibrations in the ground in September, said Stephen Hicks, study co-author and a seismologist at University College London.

It wasn’t the rich orchestra of high notes and rumbles you’d expect from an earthquake, but more of a monotonous hum, he told CNN. Earthquake signals usually last for minutes; this one lasted nine days.

He said he was astonished and that it was “completely unprecedented”.

Seismologists traced the signal as far as East Greenland but were unable to locate it precisely, so they contacted colleagues in Denmark who had received reports of a tsunami triggered by a landslide in a remote part of the region called Dickson Fjord.

The result was a nearly year-long collaboration between 68 scientists from 15 countries who sifted through seismic, satellite and ground data, as well as simulations of tsunami waves, to solve the mystery.

What happened is called a “cascading danger,” Svennevig said, and it all started with man-made climate change.

The glacier, at the base of a massive mountain rising nearly 4,000 feet above Dickson Fjord, has been melting for years, like many glaciers in the rapidly warming Arctic.

As the glacier thinned, the mountain became increasingly unstable until it finally collapsed on September 16 last year, sending enough rocks and debris crashing into the water to fill 10,000 Olympic-sized swimming pools.

The resulting mega-tsunami – one of the most powerful in recent history – triggered a wave that remained trapped in the winding, narrow fjord for over a week, lapping back and forth every 90 seconds.

The phenomenon, called “soapy,” is the rhythmic movement of a wave in an enclosed space, similar to water splashing back and forth in a bathtub or cup. One of the scientists even tried (and failed) to recreate the impact in his own bathtub.

Although seiches are well known, scientists had no idea that they could persist for so long.

“If I had said a year ago that a seiche could last nine days, people would have shaken their heads and said that was impossible,” says Svennevig, who compared the discovery to suddenly finding a new color in the rainbow.

It was this seiche that generated the seismic energy in the Earth’s crust, the scientists found.

It may be the first time scientists have directly observed the effects of climate change “on the ground beneath our feet,” Hicks said. And no place was immune; the signal traveled from Greenland to Antarctica in about an hour, he added.

No one was injured in the tsunami, although it washed away centuries-old cultural monuments and damaged an empty military base. But this stretch of water lies on a frequently used cruise ship route. If anyone had been there at the time, “the consequences would have been devastating,” write the authors of the study.

East Greenland has never experienced a landslide and tsunami like this before, Svennevig said. This shows that new areas of the Arctic are “vulnerable” to such climate events, he added.

As the Arctic continues to warm – the region has warmed four times faster than the rest of the world in recent decades – mega-tsunamis triggered by landslides could become more frequent and deadly.

In June 2017, a tsunami killed four people and washed away houses in northwest Greenland. The threat goes beyond Greenland, Svennevig said; similarly shaped fjords exist in other regions, including Alaska, parts of Canada and Norway.

The events in Greenland last September “demonstrate once again the progressive destabilization of large mountain slopes in the Arctic due to increased global warming,” said Paula Snook, a landslide geologist at the Western Norway University of Applied Sciences who was not involved in the study.

The recent rock avalanches in the Arctic and Alpine regions are “an alarming signal,” she told CNN. “We are thawing the ground that has been in a cold, frozen state for many thousands of years.”

There is still a lot of research needed on rock avalanches, which are also influenced by natural processes, warned Lena Rubensdotter, a researcher at the Norwegian Geological Survey who was also not involved in the study.

However, she added, it is “logical to assume that as the climate warms in the Arctic regions, we will see more frequent rockfalls on permafrost slopes.”

The discovery of seemingly unnatural natural phenomena shows how this part of the world is changing in unexpected ways, Svennevig said.

“It’s a sign that climate change is pushing these systems into uncharted waters.”