Holes the size of city blocks are forming in the Arctic seafloor

An autonomous underwater vehicle is recovered after completing a seafloor mapping mission in the Arctic Ocean. The remotely operated vehicle (foreground) is used to conduct visual surveys of the newly mapped seafloor.

(CNN)Marine scientists have discovered deep sinkholes -- one larger than a city block of six-story buildings -- and ice-filled hills that have formed "extraordinarily" rapidly on a remote part of the Arctic seafloor.

Mapping of Canada's Beaufort Sea, using a remotely operated underwater vehicle and ship-mounted sonar, revealed the dramatic changes, which the researchers said are taking place as a result of thawing permafrost submerged underneath the seabed.
The changes the scientists observed occurred between 2010 and 2019, during which four mapping surveys had taken place, covering an area of up to 10 square miles (26 square kilometers).
    Mapping autonomous underwater vehicles detailed the unusually rough seafloor terrain along the edge of the continental shelf in the  Arctic. Sinkholes have developed where permafrost at depth has thawed.
    It's the first time an area of submerged permafrost, a frozen layer of Earth's surface, has been surveyed in this way, and it's not known how widespread similar changes are elsewhere in the Arctic.
      On land, thawing permafrost has led to radical shifts in the Arctic landscape, including ground collapses, the formation and disappearance of lakes, the emergence of mounds called pingos, and craters formed by blowouts of methane gas contained in the permafrost. These extreme features have affected infrastructure such as roads and pipelines.
        "We know that big changes are happening across the Arctic landscape, but this is the first time we've been able to deploy technology to see that changes are happening offshore too," said marine geologist Charlie Paull, a senior scientist at Monterey Bay Aquarium Research Institute and one of the lead authors of a study on the phenomenon published Monday in the peer-reviewed scientific journal PNAS.
        "Clearly, such large changes would have significant implications for any infrastructure that might be placed on the seafloor. Currently, there is little infrastructure in this remote region of the Arctic. However, this may change as continued warming makes the region more accessible," he added.

          'Unexpected'

          About one-quarter of the land in the Northern Hemisphere is underlain by permafrost, Paull said, including large areas under the sea. This is because at the end of the last ice age around 12,000 years ago, large swaths of permafrost were submerged as glaciers melted and sea levels rose.
          In the 10-square-mile (26-square-kilometer) area of study, mapped in 2010 and again in 2019, the researchers found 41 steeply sided holes in the more recent mapping that weren't there before. The holes were roughly circular or oval-shaped and averaged 22 feet (6.7 meters) deep. The greatest change was a depression 95 feet (29 meters) deep and 738 feet (225 meters) long and 312 feet (95 meters) wide -- around the size of a city block made up of six-story buildings.
          The research team also found "numerous" hills, typically 164 feet (50 meters) in diameter and 33 feet (10 meters) high, that contained ice. They are similar to pingos -- ice-filled mounds found on land -- the study said.
          Surveys of smaller areas of the seafloor took place in 2013 and 2017, allowing researchers to understand the changes in finer detail.
          Repeated mapping surveys with autonomous underwater vehicles revealed a massive sinkhole developed over just nine years.
          Evgeny Chuvilin, a research scientist at Skoltech in Russia who has studied the Siberian permafrost, said it was surprising to see changes like these occurring in such a short span of time.
          "Permafrost degradation is a slow process. We're usually talking about centimeters per year. This here is more than merely degradation, it's also a qualitative change. So, I would say that yes, it is unexpected to see," Chuvilin, who wasn't involved in the research, said.
          "Hypotheses had been voiced in the literature concerning the possibility of such processes, but this is the first time they have been directly observed."

          Rapid changes

          Massive craters have been discovered in parts of the Russian Arctic that formed when the buildup of pockets of methane gas in the ground spontaneously exploded.
          However, the Beaufort Sea researchers ruled out a similar origin for the marine sinkholes they discovered. The team didn't find rocks and earth on the seafloor that would have been scattered by such an explosion.
          Plus, brackish (slightly salty) water near the seafloor suggested the seawater was mixed with groundwater and the submarine permafrost wasn't a sealed system where overpressure could build up. Nor did they detect significant amounts of methane in the leaking groundwater.
          "We do not have evidence that the rapid changes in this area are associated with explosive events," Paull said via email.