A team of researchers from the University of Southampton has finally solved the mystery surrounding a giant void that was first discovered in 1974 and then reappeared in 2016 and 2017.

The unusual opening in the icy continent is called polynya which closed two years by itself after its first discovery. Its size is estimated to be twice that of New Jersey.

The scientists found that the hole appeared because of the ocean’s water currents, wind, and increasing levels of salt in the water that melted the sea ice, reported Daily Mail.

The opening was called Maud Rise Polynya in the 1970s on the name of an underwater mountain in the Weddell Sea.

Such phenomena come up every year in the coastal areas, but what made it unusual was the size of it and above the thousands of feet of seawater.

Lead author of the study Aditya Narayanan, a Postdoctoral Research Fellow at the University of Southampton said: “The Maud Rise polynya was discovered in the 1970s when remote sensing satellites that can see sea ice over the Southern Ocean were first launched.”

“It persisted through consecutive winters from 1974 to 1976 and oceanographers back then assumed it would be an annual occurrence. But since the 1970s, it has occurred only sporadically and for brief intervals.”

Narayanan noted: “2017 was the first time that we’ve had such a large and long-lived polynya in the Weddell Sea since the 1970s.”

“Ekman transport was the essential missing ingredient that was necessary to increase the balance of salt and sustain the mixing of salt and heat towards the surface water,” said Alberto Naveira Garabato, the study’s co-author.

Researchers warned that polynyas can cause a rise in sea levels.