Huge landslide in Alaska caused 481m-high tsunami

A massive landslide in August 2025 caused the second-biggest tsunami ever recorded, with waters rising over 480 metres inside an Alaskan fjord.

The tsunami sped down the fjord at a speed of at least 70 metres per second and created a seiche, or reflecting wave, that sloshed around in the fjord for 36 hours.

Only the 1958 Lituya Bay tsunami, also in Alaska, which spilled over a 530-metre-tall ridge, is known to be bigger.

The region near Juneau, Alaska, is home to stunning landscapes where glaciers break off into waters enclosed by steep-sided fjords, attracting thousands of cruise ships each year.

But because the tsunami occurred at 5.26am, deep within the Tracy Arm fjord in south-east Alaska, no tourist vessels were close to where the worst of the catastrophic event took place, says Dan Shugar at the University of Calgary, Canada.

“This was a really… terrifyingly big wave,” says Shugar. “If a ship were in the upper part of the fjord, I can’t see how it would survive.”

Shugar and his colleagues reconstructed the event using satellite images, seismic data, eyewitness accounts and computer models. They found that it was almost certainly caused by a retreating glacier destabilising terrain around Tracy Arm fjord, leading to a massive landslide.

Throughout the 20th century and continuing into recent decades, the South Sawyer Glacier, which feeds into the fjord, has retreated by more than 10 kilometres and thinned dramatically.

In spite of the massive retreat, there was no significant indication that a huge section of the mountain was about to give way, dropping 64 million cubic metres of rock into the fjord. Only in hindsight did researchers identify small tremors in the days leading up to the landslide.

At 5.45am, a group of kayakers camped 50 kilometres away woke to find their campsite being inundated and their gear being swept away.

The first that Shugar and his colleagues knew about the disaster was within hours, when they were notified that the landslide had triggered a 5.4 magnitude seismic event. However, it was not until mid-October that a team could be sent to the area to study the scale of what had happened.

It could be a harbinger of a future with many more climate-change-driven tsunamis, warns Shugar. “Hopefully this will be a wake-up call to policymakers in places anywhere where we have steep landscapes next to the ocean or lakes – North America or Greenland or New Zealand, Chile – because these tsunamis are a threat that are probably underappreciated.”

“Tsunamis are generally not primarily linked to climate factors, so this is another clear example of how climate change can indirectly trigger even those natural hazards we wouldn’t traditionally associate with it,” says Martin Koehler at the University of Queensland, Australia.

“It’s fortunate that no vessels were nearby at the time, especially given the frequency of cruise ship traffic in the region and the sudden, unexpected nature of the event.”