The catastrophic collapse of the St. Francis Dam, located 80 kilometers north of downtown Los Angeles and east of the town of Santa Clarita, just before midnight on March 12, 1928, claimed more than 400 lives when towering floodwaters destroyed homes, bridges and farmland, as they swept through downstream communities. The disaster was initially blamed on the failure of the west abutment, anchored in soft conglomerate rock. Additional studies have revised this explanation, with recent research citing other geologic and design factors that likely contributed. Regardless, the collapse effectively ended the career of William Mulholland, the self-taught engineer whose 1913 Owens Valley Aqueduct made the explosive growth of Los Angeles possible. Yet, despite the magnitude of the disaster and its impact on local and national policy, it has been almost entirely forgotten, except by a few historians.