The 1963 Vaiont landslide represented, first of all, a human catastrophe for the many victims caused. Many questions, legal, economic, societal, and scientific have accompanied its history, before and after 9 October 1963. Edoardo Semenza (1927- 2002) was the geologist who discovered, some years before the beginning of the first movements, that an ancient landslide mass was present on the left side of the Vaiont Valley. In this paper, the geological and geomorphological features that led him to define the shape and the boundary of the ancient landslide, are described. Photographs from 1958 to 1963 illustrate both the complex geology of the area and those peculiar features, such as the massive aspects of the rock mass, that hindered to many, the real condition of the slope. A palinspastic reconstruction of the Vaiont slide, starting from the postglacial mass movement and including the 1963 failure, is the last contribute of Edoardo Semenza to the understanding of the event. His work helped to clarify fundamental questions regarding the existence of a paleo-landslide and its connection to the most recent movement. This paper is a personal tribute of the author to her mentor.