Dam disasters are particularly dangerous because, unlike other structures, you don't have to be on or near them to be hurt when they fail. A catastrophic collapse can send millions, or even billions, of gallons of water into communities miles downstream. It's happened dozens of times, sometimes because of poor design and other times because of just downright bad luck.
One of the United States' most infamous dam collapses happened May 31, 1889, just outside of Johnstown, Pennsylvania. Looming just upstream was the South Fork Dam, which commenced operation in 1853 and had, disturbingly, already failed twice before. Because of poor construction and some equally misguided maintenance decisions, the dam failed, sending 3.6 billion gallons of water into the communities below. When the waters receded, 2,209 people were dead [source: National Park Service].
Despite the failure at Johnstown, few standards existed when dam builder extraordinaire William Mulholland built the St. Francis Dam in San Francisquito Canyon outside of Los Angeles in 1926. The first sign of trouble appeared on the morning of March 12, 1928, when water began pouring from one side of the concrete structure. By midnight it had collapsed, sending water and debris through town after downstream town. No one knows exactly how many people perished, but some estimates are as high as 600 [source: Blitz]. Though the cause turned out to be an inherent problem with the canyon wall, and beyond the control of the engineers of the time, it nevertheless spurred dam safety legislation in California and the surrounding states [source: Association of State Dam Safety Officials].