Commonly called " The Secret Grave", it is located at the foot of the castle. Going down a flight of steps carved into the tuff, probably belonged to the ancient entrance of the castle walls, you enter an original underground environment, a sort of tomb or cave hermit. The Hypogeum is arranged on a cruciform plan, formed by orthogonal arms  to the main body; it has geometric relief decorations on the vaults. The forms used for decoration are ancient Christian symbols: it has a typical basilica plan indeed. The walls suggest a possible place of initiation of knightly orders or esoteric rituals. The cave was discovered in 1996 and opened to the public on 2 September 2016, after a careful work started in May 2012.