Failure of the Teton Dam near Rexburg, Idaho on Saturday June 5, 1976. At 350 feet, this is the tallest dam that has ever failed.
The Teton Dam was an earthen dam in Idaho, United States, built by the Bureau of Reclamation, one of eight federal agencies authorized to construct dams. Located on the Teton River in the eastern part of the state, between Fremont and Madison counties, it suffered a catastrophic failure on June 5, 1976, as it was filling for the first time. (Source)
Even more impressive, you can watch it collapse: