May 6, 2026

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A 1,500-foot tsunami took scientists by surprise. Now we know why it happened

By chance a tour boat avoided a deadly tsunami set off by the retreat of Alaska's coastal glaciers. Scientists are working to spot landslides like the one that caused the massive wave to warn people in harm's way

By Megan I. Gannon edited by Andrea Thompson

An aerial photo of the August 10, 2025, landslide and the aftermath from the tsunami it triggered in Tracy Arm taken during a U.S. Geological Survey field reconnaissance overflight on August 13, 2025.

John Lyons/U.S. Geological Survey

Early in the morning on August 10, 2025, Christine Smith awoke in a boat anchored in an inlet along southeast Alaska’s glacier-threaded coast. Smith and her husband were leading a small cruise on their 65-foot wooden boat, the David B. Inclement weather had forced them to spend the night 50 miles from their planned anchor spot in Tracy Arm, a dramatic fjord to the southeast of Juneau. As the naturalist and chef on board, Smith prepared to make breakfast and write about the rainy, foggy conditions in her daily log. Her husband, the captain, got her attention, asking, “Have you ever seen this before?”

From the boat, they could see water foaming over a nearby sandbar and repeatedly surging over, then retreating from, large rocks on the shoreline. The tide should have been falling. Perplexed, Smith texted her friend Jackie Caplan-Auerbach to see if the Western Washington University seismologist might know if this odd ebbing and flowing could be the result of a nearby landslide.

From what Caplan-Auerbach and other scientists pieced together in the hours and days that followed, Smith learned that if the David B had anchored in Tracy Arm as planned, she and the others aboard would have been killed. A loud crash around 5:30 A.M. Alaska Time would have awoken them as the north fjord wall crumbled just in front of the retreating tongue of the South Sawyer Glacier. They likely would have seen a deadly 300-foot wall of water heading their way. Their near miss underscores the threat to ships and local communities as retreating glaciers destabilize the landscape. But new research by Caplan-Auerbach and many others, published today in Science, shows that subtle clues could help provide early warnings.

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What Caused the Collapse

Over the past two decades, as the climate has continued to warm, most of Alaska’s coastal glaciers have retreated, exposing steep, U-shaped fjord walls. Researchers have been concerned that without their icy buttresses, these walls could collapse. They’ve largely focused on areas already showing signs of instability, such as a slowly degrading slope in Barry Arm in south-central Alaska, where a massive landslide could send a destructive tsunami to the town of Whittier 30 miles away in just 20 minutes. “We have our eyes on a very small number of potential slides, and there are obviously way, way more,” Caplan-Auerbach says.

But the Tracy Arm tsunami shows that not all fjords exhibit slow collapse. “At Tracy Arm, there’s no indication that that was the case,” says Dan Shugar, a geomorphologist at the University of Calgary in Alberta and lead author of the new study, which is also being presented today at the annual meeting of the European Geosciences Union in Vienna.

An animation of the landslide and tsunami in Tracy Arm.

Over the past year, Shugar and his co-authors have dug more deeply into seismic records and other observations of the glacier to figure out what happened. They found that South Sawyer Glacier has undergone long-term thinning and retreat over the last century. More acute episodes of shrinking have been documented since 2000. In the spring and summer of 2025, the ice retreated inland several hundred feet at the base of the collapsed slope, exposing most of the rock that ultimately caused the landslide just days before the sudden collapse. “It’s quite a direct link,” says Kristen Cook, a geomorphologist at University of Grenoble Alpes in France, who wasn’t involved in the study.

Using imagery and digital elevation models from before and after the slide, the researchers estimate that at least 2.26 billion cubic feet of earth collapsed, resulting in a loss of about 500 feet of coastline. They think the rupture likely extended even farther below the water’s surface but lack the bathymetry data to prove this. The resulting tsunami reached more than 1,500 feet up the side of the fjord and sloshed back and forth like bathtub water, creating a seismic “ringing” that could be detected around the