On August 10, 2025, a massive wall of rock broke loose from a slope above Tracy Arm fjord in Alaska and crashed into the water below. The remote, glacier-carved waterway — a regular stop for cruise ships — happened to be empty that day. No one watched it happen.
What followed was a wave of almost incomprehensible scale. Because there were no witnesses, figuring out exactly what occurred took scientists months of painstaking work — work that has only recently begun to reveal just how extraordinary that morning really was.
A wave taller than a skyscraper
The numbers alone are staggering. The tsunami reached 1,578 feet — 481 meters — up the steep slopes of Tracy Arm fjord, tall enough to have submerged the roof of One World Trade Center with more than 200 feet of water to spare. It ranks among the tallest tsunamis ever recorded on Earth. The only confirmed higher wave was a 1958 earthquake-triggered tsunami in nearby Lituya Bay, Alaska, which scoured slopes to 1,720 feet.
The trigger: a landslide that dropped roughly 2.1 billion cubic feet of rock into the fjord’s waters in seconds, displacing an enormous volume almost instantaneously. Despite the scale of destruction, no lives were lost. Kayakers near the mouth, miles from the impact zone, reported that strong waves washed away their equipment — evidence of how far the energy traveled — but no one was hurt.
Detective work from space and seismic sensors
With no eyewitnesses, scientists had to reconstruct the event entirely from remote data. Thomas Monahan, a senior research associate in engineering at the University of Oxford, led a team that combined satellite imagery with seismic sensor readings to build computer models of how the wave behaved.
The most valuable data source was the Surface Water Ocean Topography satellite — known as SWOT — operated jointly by NASA and France’s space agency. It captured observations of the wave’s energy as the event unfolded, providing an overhead record that ground-based instruments alone couldn’t have supplied.
Even so, the wave defied initial expectations. Satellite observations revealed it was more energetic than the computer models had predicted. That gap between model and reality is itself a significant finding — it suggests current tools may underestimate the power of landslide-triggered waves in confined waterways.
A fjord that acted like a giant tuning fork
The wave didn’t simply crash and recede. After the initial impact, water kept sloshing back and forth inside the narrow fjord in a pattern called a seiche — a standing wave that persisted for more than a day, making it only the second such long-lasting seiche ever documented. The first was observed in Greenland in 2023.
The Alaska signal proved more complex than the Greenland case, a difference the researchers attribute to Tracy Arm’s distinct geometry. The shape of a fjord determines how water resonates within it, much like the dimensions of a musical instrument determine its pitch.
“This study shows that enclosed basins like fjords can effectively act as giant tuning forks, with the resonance determined by their shape and geometry,” Monahan said in a statement. That unique acoustic-like signature gives each fjord a recognizable pattern when struck by a high-energy event — potentially useful for future monitoring efforts.
Glacial retreat and unstable slopes
Tracy Arm is the outlet for the South Sawyer Glacier, which has been retreating rapidly in recent years. Whether that glacial withdrawal destabilized the slope above the fjord remains an open question. Heavy rainfall in the period before the landslide may have been an equally important factor, or the two forces may have worked in combination.
As glaciers pull back, they expose rock faces previously supported by ice, leaving slopes vulnerable in ways that are difficult to assess in advance. Tracy Arm isn’t unique in this regard. Cruise lines have already canceled visits to the fjord this year, citing the risk of another slide — a precaution that reflects how seriously both the scientific community and the travel industry are treating the possibility of a repeat event.
Warning signs that could save lives
Perhaps the most consequential finding involves what happened before the landslide, not during it. Study co-author Stephen Hicks, an Earth scientist at University College London, noted that tiny earthquakes occurred at an increasing rate in the hours and days leading up to the collapse — a signal that the rock mass was beginning to crack.
“Many seismic monitoring stations provide data in real-time,” Hicks said in a statement, “so this gives us some optimism that we can turn what we have learned into a warning system.”
The research, published May 6 in the journal Science, could inform monitoring protocols for fjords in Alaska, Greenland, Norway, and elsewhere. Real-time seismic networks already exist across many of these regions. What remains is learning to read the subtle signals they capture before a slope gives way — not after. That work is now underway.