In September 2023, seismologists across the world noticed something strange. It wasn’t an earthquake. It wasn’t a volcanic eruption. It was something that had never been seen before – and almost no one knew it was happening.
The Signal Nobody Could Explain
It began on the 16th of September, 2023, at 12:35 UTC.
Scientists around the globe picked up an unusual oscillating seismic signal that peaked every 92 seconds – a pattern entirely atypical for an earthquake. The signal continued for more than a week, baffling researchers across every continent.
“We were like, ‘Oh wow, this signal is still coming in. This is completely different to an earthquake,'” said Stephen Hicks, a seismologist and co-author of the eventual study. “We called it an unidentified seismic object, or USO.”
The seismic waves raced around the world, zipping from eastern Greenland to Antarctica in less than one hour. They were detected in Japan. In South Africa. In Brazil. Scientists stared at their instruments, checked their calibrations, compared notes across continents, and arrived at the same conclusion: they had no idea what they were looking at.
An international team of 68 researchers from 40 institutions across 15 countries eventually assembled to find the answer. What they found would rewrite the record books – and issue a warning that the world is only beginning to take seriously.
The Mountain That Fell Into the Sea
Scientists eventually tracked the source to Dickson Fjord – a narrow channel in eastern Greenland walled in by cliffs that tower roughly 3,000 feet above the water.
Fresh satellite images, compared against images taken weeks earlier, revealed something startling: a section of mountain had simply vanished. A vast scar of bare rock marked where it had stood.
On 16 September 2023, a 25-million-cubic-metre rockslide occurred on a mountain peak – rock and ice that had been held in place for millennia by a glacier at its base.
Climate change had been quietly melting that glacier for years, withdrawing the frozen buttress that kept the mountain stable.
When the support finally gave way, roughly 33 million cubic yards of rock and ice – equivalent to the volume of 25 Empire State Buildings – came crashing down, triggering a 650-foot-tall wave that had nowhere to go in the narrow fjord.
The initial wave was approximately 200 metres high. Half the height of the Empire State Building. Moving at the speed of a jet aircraft.
The surge barrelled down the two-mile corridor, bounced off the headland, and tore back again, wrecking roughly $200,000 in equipment at an empty research post on Ella Island. The base – a Danish military research station – had been closed for the season. There were no people inside.
The Seiche: When a Wave Refuses to Die
Here is where science becomes almost difficult to believe. In a conventional tsunami, the wave propagates outward into the open ocean, dissipating its energy across thousands of kilometres.
Dickson Fjord is not open ocean. It is a narrow, enclosed channel – a natural trap. And so the wave did something extraordinary.
The water did not calm after the first pass. Instead, it began rocking from wall to wall – a motion known as a seiche. Computer models later showed the surface rising as much as 30 feet, then sinking the same amount, in a steady rhythm that pressed on the seafloor like a giant piston.
A seiche (pronounced “saysh”) is essentially a standing wave – a body of water oscillating in a closed basin, like water sloshing in a bathtub after a sudden disturbance.
In swimming pools and harbours, seiches are minor curiosities. In Dickson Fjord in September 2023, the seiche was a 200-metre wave bouncing back and forth every 90 seconds for nine consecutive days.
This movement of a large mass of water caused vibrations throughout the Earth, shaking the planet and generating seismic waves observed worldwide. Never before had scientists observed such an unusual mechanism causing a global seismic signal.
The planet, in other words, was ringing. Like a struck bell, it vibrated – not for seconds or minutes, but for more than two hundred hours – carrying the signature of a wave in a remote Arctic fjord to every seismometer on Earth.
The Lucky Emptiness
No one was injured in the event in the uninhabited Dickson Fjord. Only the Sirius military base on the small island of Ella Ø, which had been abandoned for the season, was destroyed.
That word – “abandoned” – carries more weight than it might appear to. Dickson Fjord sits near a popular cruise route. Though no passengers were present at the time of the event, the episode highlights the rising risks as Arctic travel grows.
Luxury expedition cruise ships regularly navigate these waters, bringing passengers to witness the same glaciers whose retreat is now destabilising the mountains above them. The irony is almost too precise: the retreating ice that tourists travel thousands of miles to see is the same force removing the geological glue that holds the cliffs together.
Three days after the landslide, a Danish naval boat passed through the fjord and did not notice anything unusual – the wave had diminished to the point of invisibility at the surface, even as it continued to shake the crust of the Earth below.
A 200-metre wall of water, and then nine days of planetary trembling. Detected by instruments on every continent. Seen by no human eye.
It Wasn’t the Last Time
What makes the Dickson Fjord event not merely historical but urgently contemporary is what happened less than two years later, on the other side of the Arctic.
Early in the morning of 10 August 2025, more than 64 million cubic metres of rock collapsed into Tracy Arm fjord in Alaska – nearly three times the volume of the Greenland rockslide! The landslide was preconditioned by glacial retreat caused by climate change.
The resulting megatsunami reached a runup height of 481 metres – the second highest tsunami ever recorded in human history, following only the 1958 Lituya Bay event in Alaska, which reached approximately 530 metres!
“The fact that the landslide occurred this early in the morning was unbelievably lucky. Next time – and there will be a next time – we may not be so lucky,” said Dan Shugar, a University of Calgary geomorphologist and lead author of the study.
Tracy Arm is not a remote research outpost. The Tracy Arm fjord alone sees upwards of 500,000 visitors per year. At least six cruise lines, including Carnival Cruise Line, have altered their itineraries in Alaska for 2026 due to the hazards that remain in the fjord.
With fjord regions increasingly visited by cruise ships, and climate change making similar events more likely, this unanticipated, near-miss event highlights the growing risk from landslides and tsunamis in coastal environments.
Two megatsunamis. Two remote Arctic fjords. Two near-misses involving cruise routes. Both triggered by the same cause. Both within two years of each other.
The Invisible Architecture of Risk
The deeper lesson of Dickson Fjord – and of Tracy Arm – is not about the waves themselves. It is about what is holding the mountains up, and what happens when it melts.
The trigger that unleashed the megatsunami was a melting glacier that destabilised the mountain at its base.
The investigation using satellite and ground imagery tracked the cause of the seismic activity to climate change withdrawing the frozen support that had kept the rock face stable for thousands of years.
The research team warns that man-made global warming and the resulting melting of glaciers and permafrost will make events of this kind increasingly frequent in the future.
The mountains of the Arctic and sub-Arctic are threaded with ice that has served as structural support for millennia.
As that ice retreats, slopes that were stable become unstable. The question is no longer whether more collapses will happen, but where and when.
“Climate change is driving the emergence of unprecedented extremes, particularly in remote regions like the Arctic, where our ability to monitor conditions using traditional physical sensors is limited,” said Thomas Monahan of the University of Oxford.
The good news (and there is some) is that the Tracy Arm event offered something the Greenland event did not: researchers found the landslide was preceded by several days of increasing microseismic activity, raising the possibility that future collapses in glacier fjords could be detected before failure. The mountains may give warning. The question is whether anyone is listening.
Nine Days of Planetary Memory
The megatsunami in Dickson Fjord is, in the end, a story about scale – about the gap between human perception and geological reality.
A wave two hundred metres tall formed in one of the most remote places on Earth, was witnessed by no human being, destroyed an empty military base, and then spent nine days shaking the planet so insistently that seismologists on six continents noticed something was wrong. The Earth registered what humans could not see.
“When we set out on this scientific adventure, everybody was puzzled and no one had the faintest idea what caused this signal,” said Kristian Svennevig of the Geological Survey of Denmark and Greenland.
“All we knew was that it was somehow associated with the landslide. We only managed to solve this enigma through a huge interdisciplinary and international effort.”
That effort – 68 scientists, 40 institutions, 15 countries, satellites, seismometers, and supercomputers – eventually produced an answer. And the answer is this: the planet sent a nine-day signal.
A tremor that circled the globe continuously, measurable from Antarctica to Alaska, generated by a wave that no ship, no camera, and no human eye ever witnessed directly.
The next time the mountains fall into the fjords, we may not be so fortunate as to have the audience absent.
The glaciers that draw travellers to these extraordinary places are the same glaciers whose absence is making them increasingly dangerous. The Earth rang like a bell. Whether we choose to listen is another matter entirely.
Sources and Further Reading:
- Science – A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 days (peer-reviewed, original study)
- Science – A 481-meter-high landslide-tsunami in a cruise ship–frequented Alaska fjord (peer-reviewed, 2025)
- Smithsonian Magazine – A Mysterious Seismic Signal Lasted Nine Days
- Science.org News – A tsunami in a remote fjord rang Earth like a bell for 9 days
- Live Science – Mysterious mega-tsunamis that shook the entire world for 9 days revealed by satellite
- Earth.com – 650-foot mega-tsunami captured by satellites
- ScienceAlert – A 481-Meter Tsunami Struck Alaska, And It Was a Terrifying Near Miss
- gCaptain – Alaska Mega-Tsunami Raises Alarm for Cruise Ships in Glacier Fjords
- Popular Science – A 1,578-foot tsunami struck a popular Alaskan cruise destination
