Ignoring its madness cost 2,500 lives.
The story begins in 1943 when Sade, the state electricity board, "persuades" 13 of the 35 public works ministers to vote for a new dam. The project, to generate power for Milan and Turin, goes ahead despite the fact that fewer than half the politicians have said yes. The chosen site is the narrow Vaiont valley in the Dolomites. Its inhabitants do not know about the dam until the police arrive to "persuade" them to sell their fields to Sade.
In 1956 geological testing reveals Toc's tremble, but the project goes on.
The locals ask for a footbridge across the proposed reservoir. They are turned down. Why? Because the mountain is unstable. A journalist called Tina Merlin warns of the possibility of Toc slipping into the reservoir.
She ends up in court, accused of being a communist agitator, stirring up the public against Sade.
The building of the dam goes on. In 1959 the project is completed. The world's highest thin arch dam now looms over the town of Longarone. The reservoir starts to fill. Stones fall from Toc and seismographs on the dam reveal small landslips. In 1960 there is a substantial landslide. The problem can no longer be ignored, but Sade is in a rush now as it needs to get paid for the dam before it is nationalised. Engineers try to control the moving mountain by raising and lowering the water in the reservoir and the project goes on.
By 1963 Toc is cracking up and producing weird roars. The water in the reservoir keeps going brown. The villagers write to the government expressing their fears of a landslide. On October 9, 1963, Toc starts to rumble. More than 250 million cubic metres of rock fall into the reservoir, creating a 250m wave that swamps the dam and pours down on Longarone. The dam remains intact, but the people are dead. Three engineers go to prison for manslaughter.