It was the greatest flood of the past two million years, and it posed a giant scientific riddle. A maverick geologist became convinced that thousand-foot-deep floodwaters had scoured out vast areas of the American northwest near the end of the last ice age. Mainstream scientists scorned his theory while he searched patiently for answers to what could have triggered such an inconceivably violent event. Finally, an ingenious solution silenced the skeptics: traces of an enormous ice dam half a mile high, which had blocked a valley in present-day Montana and created an enormous lake behind it. With the help of stunningly realistic animation, NOVA takes viewers back to the Ice Age to reveal what happened when the dam broke, unleashing a titanic flood that swept herds of woolly mammoth and everything else into oblivion.