Diga di Mattmark, Hydroelectric dam in Saastal Valley, Switzerland.

The Mattmark Dam is a hydroelectric structure in Saastal Valley that rises 120 meters high with a crown measuring 780 meters long. The structure holds a reservoir containing 101 million cubic meters of water.

The dam was built during the 1960s and was marked by a glacier collapse on August 30, 1965, that killed 88 workers. This tragedy became one of the most severe construction accidents in Swiss industrial history.

On 30 August 1965, part of the Allalin glacier broke off and fell onto the workers' barracks at the construction site, killing 88 labourers, almost all of them immigrants. The dead were remembered in a memorial at the dam, and the disaster shaped Swiss debate on the safety of migrant construction crews.

The site is best reached from Saastal and offers viewpoints overlooking the reservoir. Visitors should expect harsh weather conditions at this altitude, especially during colder months.

The dam's crest runs 780 metres, unusually long for a stone structure 120 metres high. Its spillway can discharge 150 cubic metres of water per second, and the reservoir it holds, the Mattmarksee, reaches 2,197 metres above sea level and stretches 3 kilometres.