Vajont Dam Rockslide

In 1963, a dam near Vajont in the north of Italy filled up following days of rain which caused a layer of clay and shale to become saturated and eventually give way, bringing down 200 million tons of limestone (a permeable sedimentary rock) down with it into the reservoir at speeds of 100km/hr in less than a minute! The slope failed because the groundwater was unable to escape to the valley floor due to the lake so it slid off the face of the mountain and into the new lake. The rockslide displaced a huge amount of water with some of it destroying a village after it swept up the opposite bank. The waves killed more than 2,600 people. Apparently, before the dam and reservoir was built, surveys failed to notice the thin layer of shale and clay that was sandwiched into the limestone.