A mega tsunami in Alaska last year reached 481 meters, warning of risks from glacier retreat and coastal rockslides. This event highlights potential dangers for cruise lines operating in the region.
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A mega tsunami in Alaska last year in a fjord visited by cruise ships is a stark warning of the risks of coastal rockslides and glacier retreat fueled by the climate crisis, a new study warns.
Scientists recorded the world’s second-tallest tsunami after it struck the Tracy Arm fjord in south-east Alaska last August after a massive rockslide around the toe of a glacier. The tsunami reached 481 metres (1,578ft) in height; by comparison the Eiffel Tower is 330 metres (1082ft).
According to the new research published in Science on Wednesday and led by Dan Shugar, a geomorphologist of the University of Calgary, the sequence began at 5.26am local time on 10 August 2025. A large landslide collapsed 1km vertically onto the South Sawyer glacier and into the narrow, 48km fjord, producing the huge tsunami.
An oblique aerial photograph of Sawyer Island, largely stripped of trees, taken during a US Geological Survey reconnaissance flight on 13 August 2025. Photograph: John Lyons/U.S. Geological Survey.
There were no fatalities at the early hour but the area is visited by approximately three cruise ships passing through daily, along with other vessels traveling within a few kilometers of the landslide site.
Just hours after the landslide, a sightseeing vessel from Juneau and a National Geographic tour boat – each capable of carrying more than 100 passengers, were due to enter the fjord. The day before, two cruise ships carrying thousands of passengers had already visited the area, with another scheduled to arrive the following day.
At the time of the event, Dennis Staley from the US Geological Survey called the tsunami “a historic event”, adding to the Guardian: “I feel like we dodged a bullet.”
“With fjord regions increasingly visited by cruise ships, and climate change making similar events more likely, this unanticipated, near-miss event highlights the growing risk from landslides and tsunamis in coastal environments,” researchers said in their report.
The mega tsunami was caused by a massive rockslide that collapsed onto the South Sawyer glacier, triggering the tsunami in the Tracy Arm fjord.
The tsunami reached a height of 481 meters (1,578 feet), making it the second-tallest tsunami recorded in the world.
Glacier retreat increases the risk of coastal rockslides, which can pose significant dangers to cruise lines operating in affected fjords.
The mega tsunami occurred on August 10, 2025, at 5:26 AM local time.
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They also noted that the tsunami was only slightly smaller than the world’s tallest, recorded in Lituya Bay, Alaska, in 1958 at 530 metres (1,728ft). The Tracy Arm event also triggered a 36-hour seiche – a standing wave that oscillates within a closed body of water.
The study further found that the landslide generated long-period seismic waves equivalent to those of a 5.4 magnitude earthquake.
Eyewitness accounts in the report highlighted the tsunami’s far-reaching effects. A group of kayakers camping on Harbor Island, about 55km away, reported water surging past their tent, sweeping away one of their kayaks along with other gear.
Another observer aboard a motor vessel in No Name Bay, roughly 50km from the landslide, described seeing a 2 to 2.5 metre wave cresting along the shoreline from the direction of Tracy Arm, followed by a second wave of about 1 metre, the researchers said.
In the study, researchers found that landslide-generated tsunamis can “have substantially higher runups (the maximum height water reaches on a slope) than earthquake tsunamis, owing to larger, localized variations in water depth and direct water-column displacement by slope failure – most pronounced in confined water bodies like fjords”.
Pointing to climate crisis-driven glacier retreat, researchers noted that “without the rapid glacier retreat, the landslide would likely not have resulted in such a wave because it would have collapsed entirely onto glacier ice or might not even have occurred at all”.
In recent years, fjords with retreating tidewater glaciers have become increasingly popular destinations for cruise ships. According to the study, annual cruise passenger numbers in Alaska have risen from about 1 million in 2016 to 1.6 million in 2025.
Aerial photographs show a clear trimline along the far side of the fjord where a tsunami stripped vegetation from the slopes. Photograph: John Lyons/U.S. Geological Survey.
Combined with accelerating glacier retreat and permafrost degradation driven by the climate crisis, the risk of large-scale landslide-generated tsunamis is also increasing across the Arctic.
As a result, researchers emphasized both the scale and potential reach of such events. They called for stronger risk mitigation measures, including systematic monitoring of unstable slopes, more realistic tsunami-modeling scenarios and enhanced protection for local communities, tourists and critical infrastructure.
Several tsunamis have occurred in Alaska over the last decade, with a large landslide generating a 18 to 55 metre wave in Kenai fjords national park in 2024, as well as another landslide near a receding glacier in Taan fjord in south-east Alaska that caused a 193 metre tsunami in 2015.