A couple decades ago, the father of a friend of mine told me the story of the day that a landslide collapsed a mountainside into a reservoir and caused 2000 deaths. The landslide instantly filled the reservoir with rocks and debris, and caused a dam to overtop with a 250 meter wave that destroyed the villages in the valley below. He lived a dozen or so miles downstream, and spent some days fishing bodies out of the river.
We toured the site of the catastrophe. As I recall, he explained that the landslide was predicted, and that the father-son engineering team that architected the dam disagreed on the likelihood and consequence of a landslide.
At the time I was there, 40 or so years after the disaster, you could clearly see how the wall of water had scoured the valley hundreds of feet above the valley floor as it washed up one side of the valley and the other on its way downstream, destroying villages along its path.
The search term 'Vajont Dam' tells the story.