There is a controversial theory that for millions of years the Earth was entirely smothered in ice, up to one kilometre thick. The temperature hovers around -40ºC everywhere, even in the tropics and the equator. If it did, then virtually nothing could survive this ferocious climate. There are some tantalising geological clues that show this theory may be true but the problem is, the clues and the Snowball Earth theory defy the laws of nature.
BBC Horizon Documentary Snowball Earth