The Vajont Dam (or Vaiont Dam) is a disused dam, completed in 1959 in the valley of the Vajont River under Monte Toc, 100 km north of Venice, Italy. A 1963 landslide caused the overtopping of the dam and around 2,000 deaths.
One of the tallest dams in the world, it is 262 m (860 ft) high, 27 m (89 ft) thick at the base and 3.4 m (11 ft) at the top. Its 1963 overtopping was caused when the designers ignored the geological instability of Monte Toc on the southern side of the basin. Warning signs and negative appraisals during the early stages of filling were disregarded, and the attempt to complete the filling led to a landslide, which created a wave that brought massive flooding and destruction to the Piave valley below, wiping out several villages completely.
On 9 October 1963 at 10:39 pm, the combination of 'drawing-down the reservoir' and recent heavy rains triggered a massive landslide of about 260 million3 of forest, earth, and rock, which fell into the reservoir at up to 110 km per hour (68 mph) which completely filled up the narrow reservoir in front of the dam. The resulting displacement of water caused 50 million3 of water to overtop the dam in a 250-metre-high (820 ft) wave. Despite this, the dam's structure was largely undamaged — the top meter or so of masonry was washed away, but the basic structure remained intact.
The flooding from the huge wave in the Piave valley destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing around 2,000 people and turning the land below the dam into a flat plain of mud with an impact crater 60 meters deep and 80 meters wide. Many small villages in the territory of Erto e Casso and the village of Codissago, near Castellavazzo, were largely wrecked. Estimates of the dead range from 1,900 to 2,500 people, and about 350 families lost all members. Most of the survivors had lost relatives and friends along with their homes and belongings.
The villages near the landslide along the lakefront also suffered damage from the air displacement caused by the impact which was so intense (calculated to be twice the air displacement caused by the nuclear bomb that was dropped on Hiroshima, Japan) that it shredded the clothes of the people in the street and caused significant damage to skin.
On 12 February 2008, while launching the International Year of Planet Earth, UNESCO cited the Vajont Dam tragedy as one of five "cautionary tales", caused by "the failure of engineers and geologists".