Enormous craters measuring 90 feet in depth have appeared on the seafloor of the Arctic Ocean.
The craters, scientists say, are forming as a result of thawing submerged permafrost on the edge of the Beaufort Sea in northern Canada, with retreating glaciers from the last ice age driving the change and not recent climate warming.
Permafrost is ground that is permanently frozen—in some cases for hundreds of thousands of years. In the Arctic, which is warming faster than any other region of Earth, permafrost is thawing, causing the ground to become unstable.
As the soil thaws, organic matter trapped within starts to break down, causing the release of methane and other greenhouse gasses. As these gasses are released, pressure builds.
On land, the impact is clear. In Siberia, there is footage showing the land wobbling "like jelly" beneath people's feet.
Eventually, when the pressure reaches a tipping point, the land explodes, leaving massive craters behind. One person who witnessed this happening described it as being "as if the earth was breathing."
What happens when permafrost on the bottom of the sea thaws is less clear, however.
In 2019, scientists in Siberia discovered a patch of ocean where the sea was "boiling" with methane, with concentrations of the gas around seven times higher than the global average.
Two years earlier, a different team of researchers found evidence of huge craters—some over 3,000 feet wide—on the floor of the Barents Sea, north of Norway and Russia. They said these craters had formed as a result of methane explosions that took place thousands of years earlier.
To better understand what impact thawing permafrost is having beneath the ocean, researchers led by Charles K. Paull, a senior scientist at California's Monterey Bay Aquarium Research Institute, used advanced mapping technology to observe changes to the seafloor over the course of a decade.
They conducted surveys in the Beaufort Sea between 2010 and 2019 to map topographical changes resulting from thawing permafrost.
Findings showed that at depths between around 400 and 500 feet, huge depressions with steep sides were forming. The largest was 90 feet deep. Their findings are published in scientific journal PNAS.
Paull told Newsweek they were shocked at their findings, with the craters far larger than they had anticipated.
He said the team does not believe the craters formed in explosive events: "The evidence suggests that the submarine features we observed forming are essentially sink-holes and retreating scarps, collapsing into void space left behind by the thawing of ice-rich permafrost."
Unlike terrestrial permafrost, climate change is not driving the seafloor to thaw. Instead, the shift is the result of older climatic shifts relating to the end of the last ice age, around 11,700 years ago. Heat is being carried to the permafrost via slow-moving groundwater systems.
The team plans to return to the Arctic this summer to look more closely at the decaying seafloor permafrost.
Julian Murton, Professor of Permafrost Science at the U.K.'s University of Sussex, who was not involved in the study, told Newsweek he was surprised at how quickly the seafloor topography had changed.
"Some changes are as rapid or even more rapid than the better-known landsurface topographic changes driven by thaw of ice-rich permafrost in the Arctic," he said. "I had assumed that thermal inertia associated with thick relict permafrost and with overlying seawater led to slow changes in seafloor topography.
"Clearly this assumption is shown to be wrong, at least locally, by this fascinating, high-resolution study."
Paull said the longer term consequences of seafloor permafrost thaw is unclear: "Since some methane is trapped in permafrost, thawing permafrost inevitably releases methane, an important greenhouse gas," he said.
"However, we don't have data to understand whether the rate of methane release from decaying submarine permafrost has changed in recent times in this area.
"The changes we've documented derive from much older, slower climatic shifts related to Earth's emergence from the last ice age, and appear to have been happening along the edge of the permafrost for thousands of years. Whether anthropogenic climate change will accelerate the process remains unknown."
This article has been updated to include quotes from Julian Murton.
