A massive hole the size of Lake Superior has opened in the ice that covers Antarctica’s Weddell Sea, a phenomenon that hasn’t been seen since the mid-1970s. This hole, called a polynya, opens up 80,000 square kilometers (31,000 square miles) of ocean in the middle of the Weddell Sea’s ice pack, hundreds of miles from shore. "This is hundreds of kilometers from the ice edge. If we didn’t have a satellite, we wouldn’t know it was there," explains professor Kent Moore, an atmospheric physicist at the University of Toronto.

The formation was first discovered in 1974, and re-opened each winter for three consecutive years, but closed and wasn’t seen again until 2016. Interestingly, it is theorized that the polynya’s 40-year absence was due to global warming: climate change effects altered the Antarctic Circumpolar Current, suppressing warm convection currents from rising from deep in the ocean, allowing the waters under the ice pack to cool.

It is currently unknown as to why the Weddell Polynya has returned, and its remoteness makes the area difficult to study. It opened for a brief few weeks in 2016, but this year’s formation has persisted since September. "This is now the second year in a row it’s opened after 40 years of not being there," according to Moore. "We’re still trying to figure out what’s going on." Moore does caution that it would be premature to blame climate change for the polynya’s recent reoccurrence, since we don’t have enough data to determine whether or not this is due to a short-term effect.

However, if the polynya does persist season-to-season, it would help exacerbate global warming: surface ice reflects a great deal of solar radiation back into space, but open water absorbs that same light, warming the oceans.

On the bright side, Moore says that the huge polynya would be akin to an oasis in the desert for marine mammals that ordinarily wouldn’t venture that far into an ice pack, so the area is probably teeming with life.