All over the world, scientists are discovering traces of ancient floods on a scale that dwarfs even the most severe flood disasters of recent times. What triggered these cataclysmic floods, and could they strike again? Over a vast expanse of Washington State called the Channeled Scablands, the level prairie gives way to bizarre, gargantuan rock formations: house-sized boulders seemingly dropped from the sky; a cliff carved by a waterfall twice the height of Niagara; and potholes resembling ones scoured out by rivers today, but ten times bigger. Like forensic detectives at a crime scene, geologists study these strange features and reconstruct catastrophic Ice Age floods more powerful than all the world's top ten rivers combined. NOVA follows their efforts to uncover the geologic fingerprints of other colossal megafloods in Iceland and—improbably—on the seabed of the English Channel, where hundreds of thousands of years ago, another deluge smashed through a land bridge connecting Britain and France and turned Britain into an island for the first time. These great disasters ripped through terrain and transformed continents in a matter of hours—and similar forces reawakened by climate change are posing an active threat to mountain communities throughout the world today.