Cause
There were many causes of the disaster. One of the main issues was the fact that the workers at the time hadn’t been able to do a proper geological survey of the area. If they had, they would have seen a large layer of gneiss below the area where the dam was built. Gneiss is an impermeable rock, preventing water from flowing through it.
Soldiers And Rescue Troops Dig in Deep Mud during Rescue Operations for Victims of the Malpasset Dam’S Collapse in Frejus On December 3, 1959. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
This meant that the water would just build up and up, not being able to flow through the ground. Later studies suggested that a tectonic fault may have been the major causes, with some even suggesting that the ground had been disturbed by explosions during the construction of a local highway.