Scientists were stunned to find something extraordinary deep down on the sea floor of the Arctic Ocean, a find they didn’t expect but one that is telling them a lot about the end of the last Ice Age. They’ve found huge craters as wide as 12 city blocks in some cases throughout the sea floor, indicating massive methane gas explosions long ago.

The craters were first spotted in the 1990s, but this latest study is the first to show just how extensive they are. Scientists believe they happened about 11,600 years ago at the end of the Ice Age as the ice sheets retreated, releasing long trapped pockets of methane gas.

And scientists have found a ton of craters, including 100 giant craters as wide as a kilometer across and thousands of smaller ones, indicating that the sea floor was quite a violent place back then.

“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.”