On September 16, 2023, scientists around the world detected a mysterious seismic signal that spread across the entire planet. It lasted for nine consecutive days.
The peak of a 3,930-foot (1.2-kilometer) high mountain in eastern Greenland held the answer.
Around 25 million cubic meters of ice, enough to fill 10,000 Olympic-sized swimming pools, crashed into the Dickson Fjord.
The impact unleashed a 650-foot (200-meter) high megatsunami wave, comparable to a 50-story building, roughly the same height as the Washington Tower in Washington, D.C.
The giant wave sped across the water, eventually reaching Ella Island, located 44 miles (70 kilometers) away off the coast, where it arrived at a height of 197 feet (60 meters).
It destroyed a research base there, fortunately abandoned at the time, causing damages worth 220,000 dollars.
The rest of the wave, about seven meters high, became trapped within the narrow, winding, and uninhabited 6.2-mile (10-kilometer) long fjord.
The wave began oscillating back and forth every 90 seconds, coinciding with the vibration recordings that traveled through the Earth's crust, producing a seismic signal of 10.88 millihertz (mHz).
This rhythmic sloshing, known as a seiche, continued its destructive path, obliterating parts of the cultural and archaeological heritage of the fjord system until it eventually diminished to just a few inches.
A Potential Catastrophe
The Dickson Fjord lies on a popular tourist cruise route, but fortunately, there were no ships nearby on September 16.
Experts agree that encountering a tsunami of that magnitude would have been catastrophic.
According to researchers who released the study and conclusions in the journal Science, the landslide was triggered by the glacier at the mountain's base losing ice, making it unable to support the rock wall above it.
Scientists are certain that similar events will happen again. In fact, it has happened before.
In 2017, a landslide in western Greenland's Karrat Fjord triggered a tsunami that flooded the village of Nuugaatsiaq, destroying 11 homes and killing four people.
"As climate change continues to melt the Earth's polar regions, the number of large-scale landslides involving earth, rock, and ice that generate destructive events will multiply," says seismologist Alice Gabriel from the Scripps Institution of Oceanography at the University of California, San Diego (UCS).
"This is the first landslide and tsunami observed in eastern Greenland, which shows that climate change is already having significant impacts there," says Dr. Kristian Svennevig from the Geological Survey of Denmark and Greenland (GEUS).
The mysterious seismic signal that triggered the tsunami was detected by seismometers worldwide, from the Arctic to Antarctica.
Its appearance was entirely different from the usual seismic noises and whistles, as it contained a single vibration frequency, like a monotone hum.
Unidentified Seismic Object
When the study's authors first discovered the signal, they labeled it an unidentified seismic object (USO).
It was puzzling for several reasons.
First, it didn't resemble the chaotic scribbles produced by earthquakes on seismographs; instead, it oscillated with 92-second intervals between peaks, which was too slow for humans to perceive.
Second, it remained strong for several days, whereas most seismic events weaken quickly.
The global scientific community began debating the possible causes of these strange seismic waves through email exchanges.
"When we embarked on this scientific adventure, everyone was baffled, and nobody had the slightest idea what was causing this signal. All we knew was that it was somehow related to the landslide," says Dr. Kristian Svennevig of GEUS.
Finally, they linked it to a massive landslide in a remote fjord in Greenland that had occurred on September 16, around the time the seismic signal was first detected.
A Lot of Work Ahead
It was the first time water movement had been recorded in the form of vibrations traveling through the Earth's crust and circulating the globe for several days.
Researchers formed a unique multidisciplinary group that involved 68 scientists from 40 institutions in 15 countries, combining seismic and infrasound data, field measurements, terrestrial and satellite images, and tsunami wave simulations.
Danish military images, taken when they sailed to the fjord a few days after the event to inspect the collapsed mountain face and glacier front, also helped reveal the dramatic scars left by the tsunami.
"This unique tsunami challenged the classic numerical models we previously used, as they simulated only a few hours of propagation," notes Anne Mangeney from Université Paris Cité and the Institut de Physique du Globe de Paris.
"We had to achieve an unprecedented numerical resolution to capture the entire event from Greenland."
The findings, researchers conclude, demonstrate the complex and cascading risks posed by climate change in polar regions.
It is now more important than ever to characterize and monitor regions once considered stable to provide early warning of these massive events.
"This shows that there are things we still don't understand and haven't seen before," concludes Carl Ebeling from the Scripps Institution of Oceanography at the University of California, San Diego (UCS).