Since our planet was born, it's gone through periods of extreme cold known as ice ages—but you might not realise just how of the cold stuff came with them. Spoiler: a lot.
This neat visualisation by Randall Monroe of XKCD fame shows just how thick the ice sheets covering our planet were 20,000 years ago. Towering 3,300 meteres above the Montreal skyline, these things were huge. So huge it's difficult to picture them, though this cartoon goes a long way in helping. Thank goodness they melted. [XKCD]