Alaska's Colossal Landslide Tsunami: A Close Encounter with a Fiord-Wall Wave

In August 2025, a dramatic event unfolded in Alaska's remote fjords. A massive landslide triggered a tsunami that sent a wave surging up a fjord wall to a height of 1,580 feet—the second tallest ever recorded. This Q&A explores the incident, its causes, and its implications for understanding such rare but devastating events.

What exactly happened in Alaska on August 9–10, 2025?

On the evening of August 9, 2025, the cruise ship Hanse Explorer was enjoying a scenic tour of the South Sawyer Glacier in Alaska's Tracy Arm fjord. Passengers captured the stunning views with selfies and videos before the ship retreated down the fjord. Just twelve hours later, a mountainside adjacent to the fjord gave way without warning. The resulting landslide plunged into the water, generating a colossal tsunami that slammed into the fjord's walls. The wave reached an astonishing height of 1,580 feet (about 481 meters)—nearly as tall as the Empire State Building. Remarkably, the Hanse Explorer had already left the area, narrowly avoiding disaster. This event ranks as the second-highest tsunami ever recorded, second only to the 1958 Lituya Bay megatsunami in Alaska.

How high was the tsunami and why is it significant?

The tsunami generated by the landslide surged to a height of 1,580 feet (481 meters) up the opposite fjord wall. This makes it the second-highest tsunami in recorded history. For context, a typical tsunami on an open ocean coastline may reach 30–50 feet; the 2004 Indian Ocean tsunami peaked at around 100 feet. The 1,580-foot runup here is extraordinary because the wave was confined within a narrow fjord, causing the water to pile up against the steep cliffs. The only larger event was the 1958 Lituya Bay tsunami in Alaska, which reached 1,720 feet. This incident underscores how local geology—specifically, steep fjord walls and unstable mountain slopes—can amplify wave heights to extreme levels. Such events are rare but potentially catastrophic for anyone in the immediate area.

What caused the landslide that triggered this tsunami?

The landslide resulted from the collapse of an unstable mountainside adjacent to the fjord. While the exact trigger isn't confirmed, scientists point to glacial retreat as a key factor. As glaciers in Alaska melt due to climate change, they remove support from valley walls, leaving slopes more prone to failure. The mountain slope that gave way had likely been weakened over decades by freeze-thaw cycles, erosion, and seismic activity. When it finally broke free, a massive volume of rock and debris plummeted into the narrow fjord. The sudden displacement of water created the towering wave. This event aligns with a broader pattern: climate-driven glacial retreat is increasing the frequency of landslides in mountainous coastal areas like Alaska, making such tsunamis more likely in the future. Monitoring and early warning systems are being developed, but the remote nature of these fjords makes prediction difficult.

How did the cruise ship Hanse Explorer survive?

The Hanse Explorer and its passengers survived because of timing and luck. The ship had finished its evening excursion at South Sawyer Glacier on August 9 and departed the fjord before the landslide struck twelve hours later. Had the ship lingered or delayed its departure, it would have been directly in the path of the tsunami. The wave's runup of 1,580 feet occurred on the fjord walls, but the main wave energy would have been devastating to any vessel in the water. The crew's decision to leave the area after sunset—standard procedure for many cruise tours—proved life-saving. This incident highlights the importance of dynamic risk assessments for ships operating in geologically active fjords. It also serves as a stark reminder that even a few hours' difference can mean survival or catastrophe in such unpredictable environments.

What are the implications for future landslide tsunamis?

This event reinforces that landslide-generated tsunamis in fjords are rare but extremely dangerous. As Alaska's glaciers continue to retreat, more slopes are becoming exposed and unstable. Scientists are now focusing on identifying hazard zones—areas where similar collapses could occur. Improved monitoring using satellite imagery, seismic sensors, and water-level gauges is underway. For local communities and maritime operators, this means developing evacuation plans and rapid notification systems. The event also provides valuable data for tsunami modeling. However, the remote locations and unpredictable nature of landslides mean absolute prevention is impossible. The key takeaway: while the odds of a ship being in the right (or wrong) place at the right time are low, the consequences are so severe that vigilance and science-based risk assessments are essential for anyone traveling in Alaska's fjords.

How does this event compare to other historic tsunamis?

The 2025 Alaska tsunami is the second-highest ever recorded. The only taller tsunami is the 1958 Lituya Bay megatsunami, which reached 1,720 feet (524 meters). Both occurred in narrow Alaskan fjords and were triggered by landslides. In contrast, most historic tsunamis—such as the 2004 Indian Ocean or 2011 Tōhoku tsunamis—are caused by earthquakes and produce far smaller runups (tens of meters). The 2025 event is also comparable to other landslide tsunamis, like the 1963 Vajont Dam disaster in Italy (though that was a reservoir failure) and the 2015 Taan Fiord event in Alaska (which reached 633 feet). What sets these apart is the amplification effect of narrow, steep-walled fjords that can turn a moderate landslide into an extreme wave. Recording such an event in the modern era provides invaluable data for hazard planning and scientific understanding.