When was the last time Antarctica was ice-free?

​​Antarctica, which is nearly four times the size of the United States, is almost entirely covered by a miles-thick layer of ice.

But the South Pole hasn't always been frozen. So when was the last time Antarctica was ice-free?

This ice cap formed relatively recently in geological terms, experts told Live Science. "I think most people would say 34 million years ago was when the ice sheet first formed in Antarctica," said Eric Wolff, a paleoclimatologist at the University of Cambridge. "[Previously] most of it would have been like northern Canada today — tundra and coniferous forest."

Global temperatures are a key factor iNFLuencing the extent of ice coverage. Around 50 million years ago, the world was about 25 degrees Fahrenheit (14 degrees Celsius) warmer than it is today, but temperatures steadily decreased over the following 16 million years. By 34 million years ago — a time period known as the Eocene-Oligocene boundary — the climate was 14.4 F (8 C) warmer than it is today.

But what triggered this temperature drop, and was that all it took for the ice sheets to form?

Related: Which is colder: The North or South Pole?

"There are two factors, and probably both were in play," Wolff told Live Science. "One of them is a change in the carbon dioxide concentration of the atmosphere, and the other is the movements of the continents and, in particular, the opening up of the Drake Passage," the strait between South America and Antarctica that connects the South Atlantic with the South Pacific.