ALASKA — A wedge of rock with a volume of at least 63.5 million cubic meters detached from a mountain above Alaska's Tracy Arm fjord at 5:26 a.m. local time on August 10, 2025, falling into the waters at the terminus of the South Sawyer Glacier. The collapse generated an initial breaking wave 100 meters high that traveled across the fjord at speeds exceeding 70 meters per second and surged up the opposite shoreline to 481 meters above sea level, with no injuries or fatalities reported.

"It was the second highest tsunami ever recorded on Earth," said Aram Fathian, a researcher at the University of Calgary. "But until now, almost nobody heard about it because it was a near-miss event."

The headscarp of the rock wedge on the northern side of the fjord stood roughly 1,025 meters above sea level. For centuries, the South Sawyer Glacier maintained the structural integrity of the slope, but the glacier has been in retreat. Between 2013 and 2022, the glacier ice bracing the failure site thinned by 100 to 130 meters, and in July 2025, glacial retreat exposed the base of the slope that later failed. According to the study, industrial-era warming made the collapse more likely, with researchers identifying a 1.1 °C increase in summertime temperatures since around 1875.

Regional seismometers registered localized repeating earthquakes beginning on August 5. By August 9, these microearthquakes were occurring approximately once every hour, and in the six hours before the landslide, the interval shrank to between 30 and 60 seconds. About one hour before the slide, seismic signals merged into a continuous grinding slip. The impact generated seismic waves equivalent in energy to a magnitude 5.4 earthquake, recorded by sensors worldwide, and sloshing water in the fjord established a 66-second long-period seiche that reverberated for 36 hours.

About 55 kilometers away on Harbor Island, kayakers saw water flowing past their tents 20 minutes after the landslide, and the surge removed some of their gear and one kayak. In No Name Bay, observers on a motor vessel reported a 2 to 2.5-meter wave followed by a 1-meter wave coming from Tracy Arm. Eighty-five kilometers from the source, the crew of a small cruise boat anchored in Fords Terror saw water pour over a sandbar, lifting their vessel three meters; the surge lasted until 11 a.m. before receding and leaving the vessel stranded.

At the mouth of the fjord, a National Geographic Venture cruise ship carrying about 150 people was anchored in dense fog. The ship's captain observed currents, white water, and ice and debris near the fjord's edges. The jagged, shallow seabed near the fjord's mouth reduced the wave's energy, and everyone on board was uninjured.

During the summer, more than 20 boats, including up to six large cruise ships, navigate the Tracy and Endicott arms daily. Cruise ship passengers visiting Alaska increased from roughly 1 million in 2016 to 1.6 million in 2025.

"It was a miraculous kind of luck we had that nobody got hurt," Fathian said. "Tracy Arm was not on the radar—it was not on anyone's hazard or risk map."

The research team aims to identify precursory warning signals detectable by seismological techniques like microearthquakes recorded days prior to the tsunami. "These signals could be promising for developing early warning systems in similar conditions or areas," Fathian said.