In September 2024, Hurricane Helene dumped more than 14 inches of rain on North Carolina, triggering flash floods and mudslides, causing over $50 billion of damage, and killing more than 100 people. A shocked nation wondered how the city that bore the brunt of the destruction – Asheville, at 2,000 feet above sea level and more than 250 miles inland – could have suffered such a fate. With eyewitness testimony, dramatic unseen videos, drone flyovers, and 3D animations, NOVA pieces together a moment-by-moment account of the Asheville disaster. And it looks to other recent flash floods – including the devastating Fourth of July 2025 floods in the Texas Hill Country, and catastrophic flooding in Valencia, Spain – for lessons that might help prevent the next disaster. Are these violent floods becoming more frequent and ferocious? And what can communities do to protect themselves in the future?