There are fears a glacier, shrinking due to climate change, might be the only thing holding back a collapsing Alaskan mountain.
Sometime in the next 20 years it's expected the side of the mountain will crash into the sea below, creating a tsunami wave potentially hundreds of metres high.
The mountain is located in Barry Arm, a fjord east of state capital Anchorage, not far from where a magnitude 7.5 quake struck on Tuesday morning (NZ time).
Cracks in the mountainside were spotted by an artist kayaking in the fjord last year, NASA's Earth Observatory said, and analysis using state-of-the-art landslide detection technology later confirmed it was "slowly and subtly shifting".
"It was hard to believe the numbers at first," said Chunli Dai of Ohio State University, when the computer told her just how big the resulting tsunami could be.
"Based on the elevation of the deposit above the water, the volume of land that was slipping, and the angle of the slope, we calculated that a collapse would release 16 times more debris and 11 times more energy than Alaska's 1958 Lituya Bay landslide and mega-tsunami."
That wave was later measured to be 524m high - it would have dwarfed the Sky Tower and almost reached the top of New York's One World Trade Centre. It was created when a magnitude 7.8 quake sent rock falling into the ocean.
Dai kept running more tests, but the more research she did, the more convinced she became the numbers were real - the narrow shape of the fjord would squeeze the wave, amplifying its height.
In May, a team put together to investigate the threat further warned in a report it might only be a matter of time.
"We believe that it is possible that this landslide-generated tsunami will happen within the next year, and likely within 20 years."
Analysis of aerial photographs showed the slip began about 50 years ago, but sped up between 2009 and 2015.
"The slope probably sped up because the glacier that had been supporting the bottom of the slope retreated," said Bretwood Higman, geologist and brother of the artist who first noticed something wasn't right. "When warming temperatures caused that ice to retreat, the slope was free to move."
Research earlier this year found landslides in Alaska happen more often in warmer years, as glaciers and areas of permafrost melt and lose integrity.
Higman told The Guardian this week there's a chance the slope could "fail catastrophically".
"If the slope doesn't fail immediately, failure is still likely as time passes, as demonstrated by repeated failures in Lituya Bay," the expert group said in their report.
"However, it is also possible that the slope will eventually stabilise for centuries or millennia."
Higman said because mega-tsunamis are rare and happen without warning, there's still a lot for scientists to learn.