The summary

• After seismologists recorded unusual vibrations, they assumed that a tsunami with a height of 200 meters had occurred in Greenland.

• The tsunami was the result of melting glacial ice, which triggered a landslide that displaced water in a Greenland fjord.

• The resulting waves echoed back and forth across the fjord for nine days.

Last September, seismologists around the world recorded vibrations the likes of which they had never recorded before. A monotonous humming sound seemed to come from Greenland. It lasted for nine days.

“At some of our stations in the north, this very, very strange signal has appeared that I’ve never seen before,” said Carl Ebeling, a seismologist at the Scripps Institution of Oceanography at the University of California, San Diego.

Shortly after the vibrations began, a cruise ship sailing near Greenland’s fjords noticed that an important landmark on the remote island of Ella – a base used for scientific research and by the Danish military for sled dog patrols – had been destroyed.

The events drew an international group of seismologists, representatives of the Danish military and oceanographers into the mystery: What had hit the island and where did it come from?

On Thursday, the researchers published their results in the journal Science. The island was hit by one of the largest tsunamis of all time, they said, with the waves leaving behind a column of water about 200 meters high.

It was the result of a series of rare, overlapping events triggered by climate change.

The first trigger, researchers found, was a collapse of the tongue of a thinning glacier caused by rising temperatures. This destabilized a steep mountainside and sent an avalanche of rock and ice crashing into Greenland’s deep Dickson Fjord, displacing a huge amount of water and causing a massive wave to sweep across the narrow fjord, which is about 2.4 kilometers wide.

The tsunami waves – some at least as high as the Statue of Liberty – ran up the steep rock walls along the fjord. Because the landslide hit the waterway at a nearly 90-degree angle, the waves bounced back and forth for nine days – a phenomenon scientists call a seiche.

“Nobody has ever seen anything like this before,” said Kristian Svennevig, the study’s lead author, a geologist and senior researcher at the Geological Survey of Denmark and Greenland.

The findings are the result of a complex, year-long investigation. The team found that the island of Ella – about 72 kilometers from the landslide – was hit by a tsunami at least 4 meters high. The island is sometimes visited by tourists.

“Just a few days before the event, cruise ships were there and moored on the beach,” Svennevig said. “We were really lucky that no one was there when it happened.”

This seiche was the longest ever observed by scientists. Until now, tsunamis caused by landslides usually produced waves that subsided within a few hours.

“It’s really a cascade of events that has never been observed before,” said Alice Gabriel, a co-author of the study. “The Earth is a very dynamic system and we are currently in a phase where this very delicate balance is being disrupted quite violently by climate change.”

Tsunamis caused by landslides are more common than many people realize and are dangerous for people living or working in some regions of the Arctic and subarctic.

In 2017, four people were killed and 11 houses were destroyed when a landslide triggered a tsunami that hit the village of Nuugaatsiaq in western Greenland. The wave was probably at least 90 meters high. Two villages were abandoned after the event because more landslides are possible. Hundreds of people remain homeless, Svennevig said.

Bretwood “Hig” Higman, an Alaska geologist who studies landslide tsunamis but was not involved in the new research, said he had compiled evidence suggesting that landslide tsunamis are a growing problem, but more studies are needed.

“I’m pretty sure these events are becoming more frequent,” he said. “Exactly how much more frequent will these events become and can we make a prediction for the future? We’re not there yet.”

Higman said he believes the researchers behind the Greenland study “hit the nail on the head” with their analysis and that the study is an important example of how dangerous these landslide tsunamis can be.

Arctic and subarctic regions are warming two to three times faster than the rest of the Earth because the darker surfaces exposed as ice melts absorb more sunlight. Warming is driving three dynamics that can make landslides more common in glaciated regions, Higman said.

First, higher temperatures cause permafrost in rock formations to erode, which can weaken slopes and make them more vulnerable to collapse. Second, warming is thinning glaciers that sometimes support rocky slopes. Removing this ice can lead to sudden collapses. Third, climate change increases the likelihood of extreme rainfall, a major risk factor for landslides, because saturated rocks and soils are more prone to landslides.

Higman is cataloging slopes in Alaska at risk of landslides that could trigger tsunamis. He said there are dozens of locations that concern him and need to be studied more closely. Some are near populated areas and could spell disaster if a landslide occurs.

“We are in a difficult position. Scientists know something, but not enough to act with certainty,” Higman said.

Last month, the U.S. Geological Survey reported a 55-foot-high landslide tsunami in Alaska’s Pedersen Lagoon. Higman visited the site and believes the tsunami was larger than originally estimated.

Globally, the risk is increasing with the expansion of development in some polar regions, leading to an increase in visits by miners, sailors and tourists, Svennevig said.

“There are more people there and at the same time the risk, the geohazard, of these landslides is increasing,” he said. “It’s an unfortunate mix.”

This article was originally published on NBCNews.com.