At 5:26 am local time on August 10, 2025, a massive wedge of rock with a volume of at least 63.5 million cubic meters detached from a mountain above Alaska’s Tracy Arm fjord. The falling rock plummeted into the deep waters at the terminus of the South Sawyer Glacier and caused an initial 100-meter-high breaking wave that tore across the fjord at speeds exceeding 70 meters a second. When this wave hit the opposite shoreline, it surged up the steep rocks to a height of 481 meters above sea level.

“It was the second highest tsunami ever recorded on Earth,” says Aram Fathian, a researcher at the University of Calgary and co-author of a recent Science study that reconstructed this event in detail. “But until now, almost nobody heard about it because it was a near-miss event,” he adds. There were no injuries or fatalities reported following the Tracy Arm fjord tsunami, mostly because it happened early in the morning. But we might not be so lucky next time.

Landslide megatsunamis

Earthquake-generated tsunamis usually reach runup heights of a few tens of meters when they strike land. Landslide tsunamis, like the one that happened in Tracy Arm, are often more localized but also way more violent. When millions of tons of rock suddenly fall into a confined body of water like a narrow fjord, the variation in water depth and the direct displacement of the water column produce extremely high waves. Since 1925, scientists have documented 27 such events with runups exceeding 50 meters. The highest was the 1958 Lituya Bay tsunami, which reached 530 meters.

The source of the 2025 Tracy Arm tsunami was a steep rock wedge on the northern side of the fjord. Its headscarp, the uppermost boundary of a landslide or rockfall, sat roughly 1,025 meters above sea level. For centuries, the structural integrity of this slope was maintained by a massive wall of ice known as the South Sawyer Glacier. But South Sawyer, like many other glaciers in the Stikine Icefield, has been in a state of retreat due to the warming climate.

Climate-driven disaster

“We studied the event from several aspects, from different lenses,” Fathian says. The team used high-resolution satellite images taken before and after the event to reconstruct the shape and geometry of the slide, as well as its axis and direction. Satellite images were also used to evaluate glacial thinning in the area, which, the team concluded, was the root cause of the Tracy Arms event.

The collapse, according to the study, was made more likely by the industrial-era warming of the planet. Researchers calculating the regional temperature trends found a 1.1° C increase in summertime temperatures since around 1875, driving up snowline elevations by roughly 169 meters. The local ice also got significantly thinner. Between 2013 and 2022 alone, the glacier ice bracing the failure site thinned by 100 to 130 meters.

Without millions of tons of ice pressing against the rock, the slopes were left too steep to support their own weight. In July 2025, just weeks before the event, glacial retreat exposed the very base of the slope that would soon fail. The icy straitjacket that kept the rocks from collapsing was no longer there. But there were other signs of an impending disaster.

Cracks in the rock

Retrospective analysis of optical and radar satellite imagery from the weeks preceding the slide showed no visible tension cracks or major deformational scarring on the slope. From the outside, it looked perfectly sound. But deep within the rock, surfaces were already grinding. Regional seismometers registered localized repeating earthquakes beginning as early as August 5. By August 9, these mini earthquakes were happening once every hour. In the six hours leading up to the main failure, the gaps between these seismic signals shrank to between 30 to 60 seconds.

The cause of this uptick in microseismicity was the small patches of rock and ice snapping as a huge part of the cliff began to inch its way downward. About an hour before the landslide, the signals merged into a continuous, grinding slip. And then, the rock fell.

The impact of 63.5 million cubic meters of rock hitting the fjord released forces large enough to be registered globally. The seismic waves that cascaded across the planet were recorded by sensor stations worldwide and were equivalent in energy to a magnitude 5.4 earthquake. The sloshing water within the fjord established a 66-second long-period seiche, a standing wave, that reverberated back and forth for 36 hours.

“It could easily turn into a catastrophic disaster,” Fathian says. It could, because Tracy Arm is a highly frequented tourist destination.

A close call

During the summer, more than 20 boats navigate the Tracy and Endicott arms every day, including up to six large cruise ships. Had the landslide occurred a few hours later, in the middle of the tourist day, the outcome could have been tragic. But even at 5:26 in the morning, the tsunami was enough to terrify the few people present in the vicinity.

About 55 kilometers away on Harbor Island, a group of kayakers saw the water flowing past their tents 20 minutes after the landslide. The surging tide took away some of their gear and one of the kayaks. Nearby in No Name Bay, observers on a motor vessel reported a 2-2.5-meter cresting wave coming along the beach from the direction of Tracy Arm, followed by a secondary 1-meter wave.

Farther away, 85 kilometers from the source, the crew of the small cruise boat anchored in Fords Terror saw a surge of water pouring over a nearby sandbar; it then physically lifted their vessel three meters despite a falling tide. The surge, they reported, lasted until 11 am, only to leave their small skiff stranded on dry land a few minutes later as the water receded.

At the mouth of the fjord, a National Geographic Venture cruise ship carrying around 150 people was anchored in dense fog. The captain noted currents, white water, and a significant amount of ice and debris near the edges of the fjord. Because the jagged, shallow seabed near the fjord’s mouth acted like a speed bump that sapped the wave’s energy, people onboard the cruise ship came out of the event unscathed. “It was a miraculous kind of luck we had that nobody got hurt,” Fathian claims.

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