In 1889, one of the deadliest disasters in American history struck Pennsylvania when the South Fork Dam collapsed and unleashed a massive wall of water on the towns below. The Johnstown Flood killed more than 2,200 people, destroyed entire neighborhoods, and changed the valley forever. This documentary tells the tragic story of the Johnstown Flood, from the failure of the dam and the warning signs that were ignored, to the terrifying destruction that followed as the flood swept through South Fork, Mineral Point, Conemaugh, Woodvale, and Johnstown. What caused the disaster? Why did the dam fail? And how did the town survive the aftermath? Watch the full story of the Johnstown Flood, the deadliest dam collapse in U.S. history.