About 260 million cubic metres of rock came off the northern face of Monte Toc into the Vajont reservoir in northern Italy on the evening of October 9, 1963, and the water it pushed aside went over the dam and down into the valley. The dam held. It is a concrete arch 262 metres high. It did not move. Longarone was erased, along with Pirago, Villanova, Faè and Rivalta, and between 1,900 and 2,000 people were killed in a few minutes. Many of the bodies were never found, because they had been ground into the debris.
Nothing was wrong with the structure, which is the point of the story. The valley walls there are limestone beds tilted toward the water with seams of clay between them that turn into slip planes when they are saturated, and filling the reservoir loaded the foot of the slope and raised the pressure of the water inside it, which is exactly the mechanism the geologists who examined the site had described in writing. The dam's builders at SADE and the state engineers who were supposed to check them went ahead anyway.
Warnings were ignored. A slide had already dropped into the reservoir during the first filling in 1960. Through September 1963 the mountain was creeping at up to a centimetre a day and then faster, and the engineers drew the water down in stages, but slowly and not far enough. On the evening of October 9 the whole flank went at once, at something like 30 metres per second, and the wave it threw up the far side of the valley reached roughly 250 metres above the crest of the dam before falling back into the Piave valley.
The trials took years and ended with light sentences, several of them cut short or wiped out by the statute of limitations; the company had been nationalised in 1962, so the state was partly judging itself. The dam still stands, empty, and the reservoir behind it is filled with the rock that came down. Vajont is taught in engineering courses not because the structure failed, which it did not, but because everyone in a position to stop it had been told what was going to happen.