It was a very early morning in August when an entire mountainside in Alaska’s Tracy Arm fjord detached and slid into the deep ocean water beneath it.
The slide created a gargantuan splash – a hyper-local, but massive tsunami that ran up the opposite mountain slope, leveling everything in its path as high as the Empire State Building. It ripped evergreens out of the ground, stripped a nearby island to bare rock and pulverized the glacial ice around it.
The whole episode lasted minutes.
About 15 miles away, a National Geographic cruise ship carrying around 150 passengers and crew started to move backward, pulled by suddenly shifting currents through an eerie fog.
And twenty miles across the fjord’s channel, three sea kayakers camping on high ground woke up to ocean water dripping into their tent, their gear strewn across the shore. One kayak was lost, swirling around in an ocean whirlpool.
It would take days for the scale of the split-second devastation to become clear, but experts say it was miraculous that no one was hurt or killed. The National Geographic cruise ship, in particular, was saved by the fact it was positioned behind an S-shaped bend in the fjord that blunted the impact of a wall of water rushing down the channel.
“That probably saved their lives,” said Jackie Caplan-Auerbach, a seismologist and professor at Western Washington University, who was in touch with the ship’s captain and relayed the tale to CNN.
But scientists who study these types of landslides are anxious it could happen again. Tracy Arm is a popular route for cruise ships packed with ice-chasing tourists eager for a glimpse of Arctic glaciers before they disappear.
“I’ve looked at the cruise ship schedules,” Caplan-Auerbach said. “On any given day, there are often a couple of cruise vessels with quite literally thousands of people aboard.”
High above the cold, clear waters of the Gulf of Alaska, the mountains are moving, inching down toward the water line.
All in all, scientists have mapped more than 1,000 instances of what they call “slow-moving landslides” in Alaska. Some are moving literal inches; others more than 10 feet per year.
Some of these mountainsides have dislodged themselves deep in the bedrock, causing the face of a mountain to shear clean off — as has happened at least twice in the last two years.
If such a enormous volume of rock, soil and debris falls into a deep body of water, whether a glacial lake or the ocean, the result is a dangerous tsunami.
Many scientists believe this phenomenon is being spurred in part by rapidly melting glaciers exposing the mountainside. Without thick sheets of ice to buttress it, the rock face becomes destabilized. Alaska has warmed 4.5 degrees since 1950, according to federal data, and is the fastest-warming US state.
But those shrinking glaciers also draw tens of thousands of tourists to the state each year. As the glaciers retreat, ships are venturing even farther into the fjords to get close to the melting giants.
Alaska’s foremost expert on these landslides knows why there hasn’t been a deadly landslide-turn-tsunami disaster, yet: sheer luck.
“It’s not because this isn’t a hazard,” said geologist Bretwood Higman, co-founder and executive director of nonprofit Ground Truth Alaska. “It’s because it just hasn’t happened to be above someone’s house or next to a cruise ship.”
The federal government and the state of Alaska are largely flying blind when it comes to tracking the landslides.
There is only one — Barry Arm, 60 miles east of Anchorage on the Alaska coast — that is continuously monitored by the US Geological Survey. Others in the area are checked periodically from satellites or aircraft. The Trump administration’s federal layoffs and budget cuts have winnowed the teams on the lookout, especially in Alaska’s heavily visited national parks. The Parks Service and USGS have been targeted for further steep layoffs by the Interior Department, according to court filings released this week.
The National Parks Service didn’t respond to CNN’s questions about whether it informs cruise ships and park visitors about these risks.
The landslide’s collapse at Tracy Arm was epic, but it wasn’t surprising to scientists.
A similar — albeit smaller — event happened just last year in Kenai Fjords National Park, when a landslide produced a 55-foot tsunami wave. In 2015, another landslide that fell near the Tyndall Glacier produced a giant wave near the Wrangell-St. Elias National Park and Preserve.
All three had one thing in common: the landslide and tsunami happened near the mouth of a retreating glacier. Some scientists, including Higman, think it’s the beginning of a worrisome trend.
“If you put it all together, it sure as heck looks like there’s been a spike in recent times,” Higman said. “If you go back to some period in the 1990s, you just find none. The level of increase is so strong that — despite all the weaknesses of this limited analysis I’ve done — I feel pretty comfortable saying that these things are happening more frequently.”
‘Throwing boulders the size of cars’
Alaska’s 1958 Lituya Bay mega-tsunami generated one of the tallest waves on record: 1,720 feet. Triggered by a strong earthquake that dumped 40 million tons of rock into the bay at once, the tsunami killed five people and wiped away two square miles of forest along the shore.
Scientists can now say with confidence that this year’s tsunami in Tracy Arm was Alaska’s second-tallest wave on record, behind Lituya Bay. The sheer height and power was “enormous,” said Patrick Lynett, a professor and tsunami modeler at the University of Southern California.
Lynett’s modeling indicates the tsunami wave was anywhere from 300 to 500 feet high, while the run-up scar it left on the mountain across the fjord reached a height of around 1,500 feet — as tall as the Empire State Building.
“What that wave looked like at the time is really hard to figure out,” Lynett said. “It could have looked like a gigantic wall of white water; it could have looked like a roaring river coming at you. It could have looked like an enormous 400-foot-high plunging wave you see surfers’ break. We don’t actually know.”
But it sounded like a freight train when it ran up onto land.
“It’s quite literally throwing boulders the size of cars and buses out of the front of it, because they’re getting entrained in the water,” Lynett said. “It’s tearing apart hundreds of feet of trees on the shorelines of either side. You can imagine the sound that must make as you’re plowing through trees on that scale.”
Sasha Caldey, 25, was the first kayaker to wake up to the roar of water outside — and the one who lost her boat to the raging ocean. “I’ve not seen such a big whirlpool before,” she said.
The kayakers said they weren’t aware major landslides were even a risk in the area.
“We heard everyone say, don’t go too close to the glaciers, because they can calve, they can flip,” kayaker Nick Heilgeist told CNN. “(A) landslide was just not in the cards.”
How to survive a landslide tsunami
The August 10 landslide happened in an area popular with cruise ships and tour boats. Had the kayaking group not called out a mayday, it would have taken longer to figure out the Tracy Arm landslide had even happened. Like the vast majority of moving mountain slopes in Alaska, it is not continuously monitored.
The one that is — Barry Arm — is a slow-moving monster: a volume of nearly 18,000 cubic feet, spanning 1.5 miles and as high as 3,000 feet. Scientists at USGS discovered it in 2020 and now use a bevy of instruments, including ground-based radar, seismometers, cameras and an infrasound array, to track how much it is moving.
Back in 2022, Barry Arm was sliding at the relatively fast clip of 8 inches each day. It slowed down in 2023 and came to a stop in 2024, according to Dennis Staley, a USGS scientist and geomorphologist specializing in landslides at the Alaska Volcano Observatory.
“We are paying the closest attention to Barry Arm,” Staley said.
Scientists are so focused on studying Barry Arm and its receding glacier in part because it is only 30 miles away from the small city of Whittier. In this growing hub of tourism and cruise ship activity, some tour boat captains say they need that data turned into warnings that can help boats and communities in the event of another tsunami.
“I’m not confident that if something happens at two minutes after 1 (o’clock) in the morning, that flow of information will get to the residents of Whittier in a timely fashion, that we’re going to be able to take action to save some people,” said Mike Bender, a local boat captain and co-owner of Lazy Otter Charters in Whittier. “Not only that, any ships that might be in transit in the area. It’s just such a short time.”
CNN reached out to National Geographic multiple times about their ship’s experience in August but didn’t receive a response.
The best-case scenario for a boat captain piloting a cruise during such a tsunami is to be in the open ocean in deep water and ride out the swell. At the Whittier-based Phillips Cruises and Tours, which offers a cruise past 26 different glaciers, Captain Cody Hanna and his crew of fellow skippers are very experienced at riding 15-foot waves created by a calving glacier crashing into the ocean.
“You want to slow down,” said Hanna, who oversees marine operations at the company. “You don’t want to drive into it full speed, and you don’t want to be broadside to it. You turn and you point into it.”
But the worst-case scenario is if a ship were to be in a narrow fjord, where there is no place to go, especially if a much bigger wave is bearing down on you.
“It’s gonna be bad news,” Bender, the Lazy Otter captain, said. “There won’t be any place to go once the wave goes up, even if you’re in deep water.”
“I’m sure it’s going to be a wild ride; all of us have talked about that,” Bender added.
Hanna wants a communication system that can transmit landslide warnings from boat to boat, so that captains can get emergency alerts even when they are out of cell service range on a tour.
“Operationally, I want as much information as fast as possible,” Hanna said. “Collecting the data is one thing and distributing it in a timely manner is another.”
Norwegian Cruise Line Holdings spokesperson Brenda Figueroa emphasized the company’s top priority is safety.
“We work closely with local, state, and federal authorities to ensure our operations meet all applicable safety standards,” Figueroa said. That sentiment was echoed in a statement from Cruise Lines International Association, a trade group that represents most major cruise companies.
Scientists agree warnings are crucial, but say much more data is needed on the slopes themselves before warnings could even happen.
When the ice disappears
The discovery of the Barry Arm landslide was a catalyzing moment. When Higman started looking around other parts of Alaska, he saw slow-moving landslides everywhere.
“It was very surprising to me how much is going on,” he said.
Many scientists point to human-caused climate change and rapidly melting glaciers as a big reason why these mountainsides are so unstable.
“The most intuitive way to describe that is, for 1,000 years prior, they had had ice holding them up at the base of their slope,” Lynett, the USC professor, told CNN. “You remove that ice relatively quickly, well, that thing that’s holding that slope in place is now gone.”
Climate change is also holding more moisture in the air, leading to heavier rainfall — which scientists say is another factor in destabilizing certain slopes.
Yet another factor they are examining is the temperature of the rocks themselves.
Rocks expand and contract when they heat and cool, said Noah Finnegan, a geomorphologist and professor at the University of California Santa Cruz. Rocks that have been covered in a layer of ice for 1,000 years have been at a consistent temperature. The sudden fluctuation they experience when the ice disappears can lead to “potentially the formation and expansion of fracture networks,” Finnegan said.
But why do some slow-moving landslides collapse and others don’t? Caplan-Auerbach is studying nearly imperceptible seismic tremors that have preceded major landslides, including the August one in Tracy Arm.
“I’m optimistic that if we had more instruments, we could learn a whole heck of a lot about this,” ideally giving communities and cruise ships hours of warnings, she said. “Do we have the capability to one day say, ‘this is an area at risk?’”
Finnegan is studying several slow-moving landslides that appear to be happening in tandem with retreating glaciers and are “strongly coupled” to the loss of ice, but “in many of those cases, the retreat of the ice doesn’t result in the catastrophic failure.”
The bottom line, scientists say, is they need more eyes and instruments on the ground to help warn people about the next landslide-generated tsunami.
“We know there are some places we should be looking, but we didn’t know (Tracy Arm),” Caplan-Auerbach said. “So, what are we missing?”
Correction: A previous version of this story included a photo caption that misstated the occupation of Paul Salopek. The story has been updated.