At the bottom of the world, fluctuations in sea ice surrounding the frozen continent of Antarctica have posed a puzzle. In West Antarctica, the vast Wilkins Ice Shelf off the Antarctic peninsula appears to be headed towards a collapse. But in East Antarctica, sea ice has been expanding since the 1970s. Researchers from the British Antarctic Survey and NASA set out to determine what was going on, and quickly ruled out one possible answer: Global warming is not an illusion, they say, and Antarctica as a whole is gradually warming up, as is the world at large. The answer to the riddle, they say, lies in a different (and almost forgotten) environmental problem: the hole in the ozone layer, which has altered weather patterns around Antarctica.