Scientists were surprised recently after finding something truly amazing under the sea at the bottom of the Arctic Ocean. Researchers found gigantic craters, some as wide as 12 city blocks, all along the sea floor, and they think that methane gas explosions thousands of years ago caused them.

The first craters were spotted back in the 1990s, but the new study reveals just how extensive the explosions were about 11,600 years ago as the Ice Age reached its end. They found 100 giant craters and thousands of smaller ones, indicating that the sea floor was a violent place when the ice sheets began to retreat and release the trapped methane gas below.

It’s an indication that we still have much to learn about the Earth during this turbulent time, with some craters stretching a kilometer in width.

“The crater area was covered by a thick ice sheet during the last ice age, much as West Antarctica is today. As climate warmed, and the ice sheet collapsed, enormous amounts of methane were abruptly released. This created massive craters that are still actively seeping methane ” says Karin Andreassen, first author of the study and professor at CAGE Centre for Arctic Gas Hydrate, Environment and Climate. “But that is nothing compared to the blow-outs of the greenhouse gas that followed the deglaciation. The amounts of methane that were released must have been quite impressive.”