The gigantic area of unfrozen water, known as a polynya, has been spotted in the ice of Weddell Sea but no one is sure why it is there.
Scientists say the enormous hole was first sighted in the mid-1970s before closing up for around four decades.
Satellite images show it mysteriously appeared again for several weeks in 2016 and it has now reopened yet again this year.
At its peak the hole measures around 31,000 square miles, nearly the size of Ireland and roughly twice the scale of the Netherlands.
Polynyas are areas of open water that are surrounded by sea ice and are usually found in Antarctica’s coastal regions.
But scientists say it is unusual to see one appearing so far in to the densely packed ice.
Atmospheric physicist Kent Moore, from the University of Toronto, told Motherboard: “It looks like you just punched a hole in the ice.
“This is hundreds of kilometres from the ice edge.
“If we didn’t have a satellite, we wouldn’t know it was there.”
Professor Moore said talk of climate change being to blame was “premature” with some suggesting it is a long-term natural phenomenon.
Polynyas are often formed by warm water rising to the surface, with the subsequent hole acting like a pressure valve to release surplus heat.
But, although the processes that lead to a polynya are fairly well understood, why this one keeps re-emerging has proved more difficult to explain.
The professor said: “This is now the second year in a row it’s opened after 40 years of not being there.
“We’re still trying to figure out what’s going on.”