Antarctic ice hole the size of Switzerland keeps cracking open. Now scientists finally know why.

Scientists have finally discovered what's causing a Switzerland-size hole to repeatedly open up in Antarctica's sea ice.

Researchers first spotted the hole, called the Maud Rise polynya, in 1974 and 1976 in Antarctica's Weddell Sea, and since then it has reappeared fleetingly and sporadically  — opening up in different sizes but in the same place, then sometimes not at all for years. This left scientists puzzled as to the exact conditions needed for the hole to form.

In 2016 and 2017, a huge 309,000 square mile (80,000 square kilometers) opened up for several weeks during both winters, enabling scientists to get a closer look at the phenomenon and finally solve the 50-year mystery. They reported their findings Wednesday (May 1) in the journal Science Advances.

"2017 was the first time that we've had such a large and long-lived polynya in the Weddell Sea since the 1970s," lead author Aditya Narayanan, a postdoctoral researcher at the University of Southampton in England, said in a statement.

When summer turns to winter in Antarctica, sea ice expands from its minimum of around 1 million square miles (3 million square km) to 7 million square miles (18 million square km), covering 4% of Earth's surface in irregular, porcelain-white tiles. 

Most of this sea ice grows during the weeks-long polar night  on the floating ice shelf that wraps around the continent. Holes in this ice, called polynyas, form when strong winds from inland push the tiles apart. 

Related: 'Unprecedented,' 'Gobsmacked', 'Unbelievable': Changes in Antarctica's sea ice could have dramatic impacts, says climate scientist Edward Doddridge