Last August, at 5:26 in the morning, a colossal mass of rock—64 million cubic metres, the equivalent of two dozen Great Pyramids of Giza—sheared off a mountain flank and plunged a kilometre into the waters of Tracy Arm Fjord in southeast Alaska. What followed was one of the most immense tsunamis ever measured: a wave that climbed the opposite slope to 481 metres, higher than the top viewing platform of Toronto’s CN Tower. The event, detailed this week in the journal Science, went largely unremarked at the time because of its remote location and the early hour. That timing, scientists now stress, was providential. Tracy Arm is a busy summer destination for cruise ships and tour boats; had the landslide struck a few hours later, the death toll could have been catastrophic.

The wave itself was not the only phenomenon. The initial splash sent a train of surges—the tallest standing 100 metres high—racing down the fjord at more than 250 kilometres per hour, their energy taking over 36 hours to fully dissipate into the open Pacific. Viewed from Washington, the episode fits a troubling pattern. Across Alaska and British Columbia, retreating glaciers are leaving behind unstable valley walls that, once buttressed by ice, now teeter without support. Analysts in London note that the Tracy Arm slide is the second-largest megatsunami ever recorded, after the 1958 Lituya Bay event in the same region, which produced a 524-metre wave. The recurrence of such extremes in less than a lifetime suggests that the risk is not static but accelerating as the climate warms.

For Canadian policymakers, the implications are immediate. Dan Shugar, a geomorphologist at the University of Calgary and the study’s corresponding author, points to towns such as Prince Rupert and Port Alberni, which sit at the heads of fjords remarkably similar to Tracy Arm. Both communities have experienced tsunami damage in the past—Port Alberni was flooded by a distant earthquake-generated wave in 1964—but the threat from local, landslide-triggered tsunamis is far less well understood. Ecological scars are also vivid: the wave obliterated centuries-old forests along the fjord’s shore, stripping vegetation and soil to bedrock in minutes. The habitat loss, Shugar warns, will take decades to recover.

Looking forward, the scientific community is urging a shift from reactive to predictive monitoring. Satellite imagery and seismic networks can detect landslides in near-real time, but few fjord systems are instrumented for the rapid wave modelling that would allow evacuation. As glaciers continue to thin and permafrost thaws, the zones of vulnerability will expand. The Tracy Arm megatsunami is not an anomaly; it is a rehearsal for a future in which such events become a regular, and far more dangerous, feature of life along the world’s glaciated coasts.