In the 1920s, geologist J. Harlen Bretz advanced a startling hypothesis that the deeply pockmarked terrain of eastern Washington was not the result of slow weathering but rather of a catastrophic flood that engulfed the area thousands of years ago. This fascinating episode of "Nova" plunges into the mystery of the ancient deluge and offers new evidence to support Bretz's once-ridiculed ideas, which made him the subject of professional ostracism.