On the night of Aug. 9, 2025, passengers aboard the Hanse Explorer were on deck photographing the South Sawyer Glacier. Twelve hours later, a landslide from the mountainside plunged into the fjord and triggered the second-tallest tsunami run-up ever recorded.
The slide dumped 64 million cubic meters of rock and debris into the water, generating an initial wave 328 feet high. Rebounding off the fjord walls, the wave stripped trees and vegetation from the opposite slope all the way up to 1,578 feet above sea level, exposing bare rock.
Only one event has surpassed it: the 1958 Lituya Bay megatsunami, triggered by a magnitude 7.8 earthquake, which reached a maximum run-up of 1,719 feet.
On May 6, 2026, the journal Science published a reconstruction of the event by an international team led by Dan Shugar, a geomorphologist at the University of Calgary. To reconstruct the event, the researchers combined satellite, aerial, and field observations with eyewitness accounts and computer simulations.
Why the Mountain Collapsed
Tracy Arm fjord is 31 miles long and barely 0.8 miles wide, with walls rising nearly 6,600 feet. That geometry proved decisive: the channel’s steep walls concentrated and amplified the force of the impact. But the trigger wasn’t seismic. It was a retreating glacier.
Rapid warming had caused the South Sawyer Glacier to retreat roughly 1,640 feet, slowly destabilizing the slope until 370 million tons of rock finally plunged into the fjord.
For decades, the ice had acted as a natural buttress, holding the rock wall in place. As the glacier retreated, it left the slope’s base unstable and exposed. In July 2025 alone, glacial retreat had exposed part of the slope’s base — the very section that would soon give way. Most of that retreat had occurred between Aug. 2 and Aug. 5 — just days before the collapse.
Unlike at Lituya Bay, Tracy Arm produced virtually no seismic signal before the slope failed. The collapse registered on sensors worldwide as the equivalent of a magnitude 5.4 earthquake — but that signal was a record of the event, not a warning. Water continued surging back and forth through the fjord for more than a day in a phenomenon called a seiche.
There were no fatalities. The ship had left the night before. A group of kayakers camped near the mouth of the fjord had their gear swept away by the surge. Meanwhile, passengers aboard a cruise ship in nearby Endicott Arm noticed sudden, sharp rises and drops in water level.
Leigh Stearns, a glaciologist at the University of Pennsylvania who was not involved in the study, emphasized how deceptive that gradual process can be: a retreating glacier seems slow and harmless, but it can unleash sudden, catastrophic events.
That makes it a hazard that’s nearly impossible to predict. Hurricanes are tracked for days. Earthquakes produce precursor seismic waves. A glacier retreating over months sounds no alarm — until the slope it was holding in place gives way all at once.
Brentwood Higman, of Ground Truth Alaska and a co-author of the study, noted that the transition from stable ice to a period of rapid glacial retreat — visible in satellite imagery — could serve as an early warning that a given area has become more vulnerable to landslides and tsunamis.
Shipping lines have suspended routes into the fjord this year. Scientists warn that other Alaskan fjords face similar conditions. At Barry Arm, roughly 100 miles east of Anchorage, a slope is already in motion.
Shugar didn’t mince words: a similar event could happen this very summer.
