Did the Earth once undergo a super ice age, one that froze the entire planet? Paul Hoffman claims that the Earth experienced just such a climate cataclysm 700 million years ago. The consequences of this idea are far-reaching. For Hoffman believes that this global deep freeze triggered the great Cambrian Explosion, the so-far unexplained moment in geological time when a glorious profusion of complex life forms first sprang from the primordial soup. In her account, Gabrielle Walker takes us on a journey to some of the most picturesque and formidable places on Earth - from the polar ice cap to the Australian outback and the African desert - as Hoffman and his fellow geologists travel the earth to find supporting evidence for their theories.