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In 2015, 76 million cubic meters of rock crashed from the rugged cliffs above a southeastern Alaska fjord and into the water below. The landslide sparked a nearly 200-meter-tall wave that roared down the narrow Taan Fiord and out into Icy Bay. No one witnessed the collapse, but a year later, geologist Bretwood Higman was in the area taking detailed measurements of the tsunami’s effects. Looking up from his work, Higman saw a massive cruise ship crossing the fjord’s mouth. He was stunned.

“It’d never occurred to me that a cruise ship would go into Icy Bay,” Higman says. An image of tsunami-tossed ships trapped in the rocky passage filled his mind. “There are many ways in which that could work out really badly.” He couldn’t get the picture out of his head.

Landslide-generated tsunamis are low-probability, high-consequence events. But as rising temperatures cause glaciers to melt, the steep slopes of southeastern Alaska’s numerous fjords are becoming increasingly unstable. Once buttressed by ice, many exposed cliffs now stand unsupported and at risk of collapse as the glaciers that once held them up rapidly retreat. Heavier rains and thawing permafrost are further increasing the hazards. And with tourists flocking to Alaska’s rugged coast, “there are now these huge concentrations of people that are going right to the areas of highest risk,” Higman says. We’ve increased our vulnerability to disaster, and we’ve increased the probability, he says. This risk is rising in coastal regions around the world that share Alaska’s conditions, such as Greenland, Chile, Norway, and New Zealand.

Unlike tsunamis triggered by earthquakes far offshore, which take time to strike coastal communities, tsunamis triggered by coastal landslides appear suddenly and can cause significantly higher waves, Higman says. That poses a greater threat to people in boats.

The growing threat has been gnawing at Amanda Bauer, who’s operated day cruises for 17 years, navigating the tight channels around Alaska’s Prince William Sound, including in the Barry Arm fjord, where a 500-million-cubic-meter slab of unstable terrain is teetering above the retreating Barry Glacier. “I think about it a lot when I’m up there—what would I do?” Bauer says. “Sometimes I’ll be sitting there, surrounded by ice; I couldn’t go more than two knots if I wanted to. That’s different than having open water where I can turn and burn if I see something happening.”

In 2017, researchers with the US Geological Survey modeled how the 2015 landslide near Alaska’s Tyndall Glacier caused a tsunami that ripped down Taan Fiord and out into Icy Bay. Animation by George et al.

Concerned about how captains should respond to such an extreme threat, Higman dove into the existing scientific literature on how ships can ride out tsunami waves. Focusing only on research related to coastal landslide-triggered tsunamis, his search turned up little, save for some one-off case studies and eyewitness accounts of historical events, such as the time in 1958 when a wave nearly the height of Toronto’s CN Tower capsized two boats in Lituya Bay, Alaska, killing two people. Scientific efforts to model landslide-generated tsunamis and their effects on vessels are just beginning, which means there’s scant data to inform guidelines.

Higman found that the official guidance from the United States’ National Tsunami Hazard Mitigation Program is similarly lacking. That advice, informed by the effects of offshore tsunamis on California harbors, essentially boils down to three bullet points: For docked vessels, abandon ship and head for high ground on foot. For vessels in deep water—between 90 and 180 meters deep—go out to even deeper water. And for vessels near shore, choose to either beach the boat and run, or flee to deeper water. This one-size-fits-all advice is meant to apply to everything from kayaks to fishing boats to 150-passenger day cruisers.

Since landslide-generated tsunamis can strike before experts are able to detect them and issue warnings, Higman says the captains he’s spoken with would never choose to deliberately beach—and potentially destroy—their vessel and attempt to evacuate with passengers and crew up a rugged Alaska shoreline without even knowing when the wave will arrive or how far it will run up the coast.

While it’s currently impossible to predict the arrival time or size of a landslide-generated tsunami in advance, Higman says guidelines could explain how tsunamis generally work. Tsunami waves differ fundamentally from the wind waves mariners are used to navigating, he says, which can throw off a captain’s intuition. For one thing, tsunami waves pick up speed in deeper water and grow considerably taller in shallow water. Since the depths of Alaska’s fjords can vary widely, a captain could think they have plenty of time to outrun a tsunami, only to have the wave catch up and break right on top of them.

Tsunamis confined to fjords also tend to slosh around like water in a bathtub, creating unpredictable currents in excess of 100 kilometers per hour. Those three bullet points of guidance don’t get into these nuances of tsunamis’ interactions with Alaska’s complex shoreline, Higman says. By oversimplifying tsunami science to such a degree, he says the guidelines also underestimate the expertise of vessel operators, who are used to making quick decisions in hazardous conditions.

Elena Suleimani, a tsunami modeler for the Alaska Earthquake Center and coauthor of the existing guidelines, admits they’re imperfect. While she’s created harbor-specific maps outlining where the water is deep enough for a ship to safely ride out a tsunami, Suleimani doesn’t feel comfortable giving advice to vessel operators: “I have no idea how to operate boats,” she says

So, on a mission to give captains the best advice possible, Higman is corunning a workshop with the Prince William Sound Regional Citizens’ Advisory Council (RCAC) in Valdez, Alaska, in June 2024. The event will bring together tsunami scientists and vessel operators for the first time to compile their knowledge and, hopefully, work out some more practicable recommendations.

At this point, Higman can’t say exactly what the proper guidance should be. But while the workshop will focus on improving advice for the captains of small craft, Chad Hults, a geologist with the National Park Service (NPS), says the operators of larger vessels, like cruise ships, need to consider the threat of landslide-generated tsunamis, as well. Hults says the NPS is keen to begin talks with the cruise lines that frequent Glacier Bay, where a dozen slabs of land seem ready to slide at any moment.

During tourism season, says Hults, “we have 260 cruise ships—two cruise ships a day—going into Glacier Bay. There’s no other place in the park system where we have 4,000 people on a boat and a pretty obvious hazard that could cause some harm.”

Similarly, says Alan Sorum, the maritime operations project manager for the Prince William Sound RCAC, there are no official tsunami hazard guidelines for the oil tankers visiting Valdez, Alaska—the endpoint of the Trans-Alaska Pipeline. “If you capsize a big vessel like that,” says Sorum, “it would be a big problem cleaning that up.”

So far, Alaska’s mariners have managed to avoid the worst. A tsunami hasn’t caused an oil spill or killed anyone aboard a boat in Alaska in 60 years. “With all my effort on this, there’s this voice in the back of my head that’s like, ‘Maybe it’s not a big deal, maybe I’m wasting my time,’” Higman says.

But then he thinks about Barry Arm, Lituya Bay, and the cruise ship he saw sailing past the mouth of Taan Fiord. He tallies the dozens of unstable slopes known to be lurking across Alaska, all waiting to collapse into bays and fjords. “And I do think that, at some point, [the situation] is going to explode.”