A large hole has opened up in the sea ice cover that sits around Antarctica, satellite images show. The hole that’s the size of the state of Maine has been there for about a month or so but researchers aren’t really sure where it came from or why it formed.

These types of sea ice features are known as polynyas. They’re open water where there should, under normal circumstances, be sea ice. They’re sometimes irregularly shaped, but usually as in this case they’re large and roughly oval shaped. The space where they form stays open because there are processes that stop new ice from forming there or the ice is moving from the region. There are two different types of polynyas and they mainly differ in how they lose their ice.

A sensible-heat polynya is formed through the transfer of heat between two entities that are two different temperatures. When this happens the body with more heat transfers some of it to the body with less heat, according to the National Snow and Ice Data Center.

In the case of a polynya, water that is warmer than sea ice comes in contact with the ice, transfers some of its heat and consequently melts some of the ice. This can happen when water above freezing temperatures upwells, or comes from lower parts of the ocean up to the top where the ice sits.

The second type of polynya is a latent-heat polynya. These occur with a change in state rather than a change in temperature. When ice melts heat is then released into the the surroundings. They can occur when winds blow in one direction constantly and push the ice away.

But this polynya was formed by a process separate from these two types of polynyas, atmospheric physicist Kent Moore told Motherboard. This polynya is deep in the ice pack which typically doesn’t happen.

As a professor at University of Toronto’s Mississauga campus Moore also works with the Southern Ocean Carbon and Climate Observations and Modeling group. The group identified a polynya in the same area of the Weddell Sea in the 1970s, that was the last time a polynya this large was observed.

The researchers were able to get information about this hole thanks to a floating array that made its way to the polynya and ended up surfacing within it and made contact with satellites. The measurements it had made were transmitted at that time and will help reveal the cause for these types of polynyas. But the research is still unpublished as it’s still in the process of being gathered and analyzed.