"Nova investigates a provocative new theory that suggests the extinction of more than 34 types of large prehistoric mega-creatures, such as the saber-toothed cat and woolly mammoth, was caused not by climate change or the arrival of the first human hunters, but by the massive breakup of a comet over the Great Lakes region at the end of the last Ice Age, some 12,900 years ago." WGBH/Nova website.