As Prime Minister, Leone had to face one of the most tragic events in Italian republican history, the Vajont Dam disaster. On 9 October 1963, a landslide occurred on Monte Toc, in the province of Pordenone. The landslide caused a megatsunami in the artificial lake in which 50 million cubic metres of water overtopped the dam in a wave of , leading to the complete destruction of several villages and towns, and 1,917 deaths. In the previous months, the Adriatic Society of Electricity (SADE) and the Italian government, which both owned the dam, dismissed evidence and concealed reports describing the geological instability of Monte Toc on the southern side of the basin and other early warning signs reported prior to the disaster.