There is a controversial theory that for millions of years the Earth was plunged into a catastrophe and entirely smothered in ice, up to one kilometre thick. The temperature hovered around minus 40 C - everywhere, even in the tropics and the equator. If this really happened, then virtually nothing could have survived 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.For over fifty years a group of scientists have been trying to prove this incredible period of Earth history. Struggling against scepticism and disbelief, now finally the many mysteries have been solved and the scientific community is slowly coming around to the extraordinary idea not just of the dramatic freeze, but of an equally dramatic thaw.From the baking landscape of Africa to ice-covered Antarctica, Horizon follows the tale of a theory which, if true, would have huge implications. Because scientists now believe this cycle of freezing and frying may have created the unique conditions needed for the evolution of complex life, including our own - and that humans only exist because of the catastrophic events of Snowball Earth.