A 500 Metre Tall Mega Tsunami Just Struck Alaska - New Photos Released

The Half-Kilometre Tall Mega-Tsunami That Struck Alaska

In the early morning hours of August 10, 2025, a remote mountainside in Southeast Alaska gave way without warning. High above the toe of the South Sawyer Glacier in Tracy Arm fjord, roughly 80 miles south of Juneau, an enormous mass of rock and soil suddenly broke loose. In an instant, tens of millions of cubic meters of mountainside collapsed and plummeted downward. The landslide—estimated at nearly 100 million cubic meters in volume, roughly 40,000 Olympic swimming pools of rock—crashed into the fjord, slamming into the icy waters. The collapse was so massive that seismometers across Alaska (and even as far as 1,000 km away) registered it as if it were an earthquake. Locals later described hearing a low rumble and cracking roar reverberate through the dawn darkness as the side of a mountain collapsed into the sea.

This was not a typical landslide of muddy soil—it was a whole chunk of mountain giving way. Geologists note that the steep glacial valleys of Alaska are prone to such giant landslides, especially as environmental conditions change. In recent decades, the South Sawyer Glacier had retreated more than a kilometre from its earlier extent, removing the support that the ice once provided to the fjord’s walls. At the same time, the region’s warming climate has been thawing alpine permafrost – the normally frozen ground that helped bind rock layers together. As the glaciers thin and retreat and permafrost thaws, the stability of these valley walls is undermined, making colossal rockfalls more likely. All these factors set the stage for what happened at Tracy Arm. The exact trigger is still being studied, but intriguingly, scientists discovered a swarm of tiny tremors (magnitude 1–2 micro-earthquakes) in the 18 hours leading up to the failure. These subtle precursors suggest the mountain was slowly starting to crack and shift before the final collapse, hinting that early warning signs might one day be detectable for such events.

This is the site of the collapse: an enormous section of the mountainside sheared off, exposing a raw scar of pale, freshly broken rock.

The Wave

When the mountainside crashed into the fjord, it displaced an unimaginable volume of water. The impact instantly heaved a towering wall of water up and outwards. Directly opposite the landslide, the wave shot up the fjord’s steep slope to an estimated 470–500 meters (1,500–1,600 ft) above sea level – nearly half a kilometre high. This is almost on par with the tallest tsunami ever recorded (the 1958 Lituya Bay mega-tsunami, 524 m). Trees and vegetation on that opposite mountainside were obliterated, scraped away by the water’s incredible run-up height. For a moment, the normally tranquil Tracy Arm was akin to a gigantic splash in a bathtub, with water surging explosively against the fjord walls.

As the tsunami wave radiated away from the landslide impact zone, it spread down the length of the narrow fjord at breakneck speed. The wave’s height diminished as it travelled, but it was still enormously destructive. Infact, we can see exactly how high the tsunami reached as vegetation has been stripped from the mountains.

A short distance away at Sawyer Island – a small island where North and South Sawyer Glaciers’ fjords meet – the tsunami was at least 30 meters (100 feet) high, stripping the island’s forested slopes bare, leaving behind only a small mat of vegetation. It was absolutely scoured. Further down-fjord, the wave continued to attenuate, but even many kilometres from the source it was still powerful. At the mouth of Tracy Arm, some 30 miles from the landslide, three kayakers camped on Harbor Island awoke around 5:45 AM to a surging flood of water rushing through their campsite. They scrambled from their tents to discover the sea had risen far above the normal high-tide line – later estimates put it at about a 3–4-metre (10–15 ft) surge at that location. Their kayaks and gear were swept away by the sudden torrent, one boat ending up wedged in a tree and another carried a quarter mile offshore. “It was just pure chaos out of nowhere,” one of the campers said later, describing the shock of seeing a wall of water appear in an otherwise calm morning.

The tsunami’s energy continued beyond the confines of Tracy Arm, spreading into broader waterways. Remarkably, the waves were detected nearly 60–75 miles away in Juneau, where a tide gauge recorded a series of oscillations about 35–36 cm (14 inches) high above normal sea level. The water in Juneau’s harbor sloshed in and out for hours afterward as the fjord’s massive splash sent long-period waves pulsing through Southeast Alaska’s channels. Scientists compared it to water sloshing back and forth in a bathtub after a big splash – the fjords and bays kept reverberating for a surprisingly long time after the initial event.

The Impact

Amazingly – and fortunately – no lives were lost in this catastrophic event. Tracy Arm is remote with no permanent settlements, and the landslide struck at 5:30 AM, a time when no tour boats or fishermen were in the fjord. “It is hard to imagine that, in front of the landslide itself, anything would survive,” one scientist noted, emphasizing how lethal it would have been had anyone been near the South Sawyer Glacier at that moment. In this case, the only human impact was the scare and minor injuries of the campers at Harbor Island (who, despite losing their equipment, were rescued safely later that day). In the aftermath, local tour operators did have to adjust – on the next day, sightseeing boats that ventured into Tracy Arm reported seeing floating logs, icebergs, and debris and had to turn back due to the hazardous flotsam in the water. Neighbouring fjords like Endicott Arm were untouched, but in Tracy Arm the evidence of destruction was everywhere: uprooted trees bobbing in the water and shorelines scoured clean.

The environment around the landslide area suffered a violent transformation. A huge section of the steep slope is now a raw scar of bright, freshly exposed rock where a lush alpine slope once stood. On the opposite side of the fjord, a matching swath of destruction marks the high-water line of the tsunami – a trimline cutting across the green forest where everything below that line was ripped away by the wave. Entire stands of trees were mowed down along the shores closest to the slide. The waters themselves turned a murky brown grey from all the pulverized rock and soil dumped into the fjord. Chunks of the South Sawyer Glacier’s ice were also sent bobbing out to sea – the landslide fell partly onto the glacier’s terminus, shearing off a portion of the ice. In effect, the event was a one-two punch: first the massive rock avalanche reshaped the landscape, then the tsunami reshaped the shoreline ecology.

This Tracy Arm tsunami appears to be one of the largest of its kind in Alaska since 2015, when a similar landslide-generated wave struck Taan Fiord (Icy Bay). That 2015 event had a run-up of ~190 m (620 ft), whereas the Tracy Arm wave of 2025 climbed roughly 470–500 m – making it perhaps the second-highest tsunami runup ever recorded globally. Only the legendary 1958 Lituya Bay mega-tsunami was higher. Scientists and historians quickly drew comparisons to Lituya Bay, where a nighttime landslide caused a giant wave that snapped trees 524 m up a mountainside. The Tracy Arm incident, while slightly lower in maximum runup, released even greater volume and energy, according to preliminary estimates. Alaskans were extraordinarily lucky that this disaster unfolded in an uninhabited fjord.

*Image shows the opposite side of the collapse where the Tsunami had a runup of 500 meters.

The Science

What caused this mountainside to give way so catastrophically? Geologists are studying the geophysical cocktail of factors that led to the Tracy Arm landslide. One clear contributor is the region’s rapidly changing climate and glacial landscape. The South Sawyer Glacier, like many others in Alaska, has been retreating and thinning dramatically. As the ice pulled back from the fjord over the past decades, it removed a physical buttress that had propped up the steep slopes. Once the glacier retreated more than a kilometre, the rocks high above lost support and were free to shift and crack. Additionally, warming temperatures are causing permafrost (permanently frozen ground) to thaw at high elevations. In many mountain areas, permafrost acts like a glue holding the rocky slopes together; when it melts, fractures and weaknesses can grow. As a state geologists’ report on a similar site noted, “as alpine permafrost thaws and glaciers thin and retreat, support of the valley walls is degraded and removed, allowing rockfalls and landslides to occur”. In short, the stage was set by years of gradual destabilization: frost no longer cementing the rocks, and ice no longer buttressing the mountain.

Yet, the final trigger on August 10 was not an earthquake or an obvious event like heavy rain. Instead, it seems the mountain was self-destructing from internal stress. The discovery of an 18-hour sequence of tiny seismic tremors leading up to the landslide is exciting for scientists. Those tremors were essentially little cracks and pops as the slope began to fail, acting as a natural alarm bell. Such precursor signals have rarely been observed before large landslides. Researchers hope that by studying this seismic signature, they might improve early-warning methods for landslide-generated tsunamis in the future. If subtle shaking can be detected and recognized in time, authorities could potentially issue warnings to get people off the water before the big slide hits.

The Aftermath

Satellite photos taken before and after the event were also compared side-by-side to map the changes. These images are striking. In the satellite view, one can clearly see the huge new landslide scar on the north side of the fjord, and directly across from it, a wide swath of lighter-coloured terrain where trees were stripped by the wave. The pre-event imagery even shows how far the South Sawyer Glacier had extended just days before; the difference highlights how rapidly the landscape was altered and how far the glacier has pulled back from its old position.

Through the dramatic USGS photos we’ve gotten a rare glimpse of what a half-kilometre-high tsunami looks like (which is something almost beyond imagining).

The Tracy Arm mega tsunami will go down as one of those rare, jaw-dropping moments when we see the full scale of nature’s power laid bare. In a matter of minutes, millions of tons of rock and ice, driven by gravity and weakened by a changing climate, managed to unleash a wave that literally reshaped a landscape. Scientists will continue to study this event for years, and agencies will continue to monitor the region, but one thing is certain: the image of that colossal wave ripping through an empty fjord, and wiping it clean, will remain a part of geologic legend.

Here's the video we made on this on the OzGeology YouTube Channel: