Roughly 35 million years ago, Earth cooled rapidly. At roughly the same time, the Drake Passage formed between South America and the Antarctic, paving the way for the Antarctic Circumpolar Current. Thanks to these two factors, Antarctica was soon completely covered in ice. This massive glaciation was delayed in at least one region.
Click here for original story, The formation of the West Antarctic Ice Sheet was very different than previously believed
Source: ScienceDaily