From Rome: Profile of a City, 312–1308 (1980)

The high altar of Constantine’s church (and later Michelangelo’s) is positioned directly over a dirt grave that was clearly thought to be Peter’s. The grave was surmounted by a pillared shrine. Archaeologists found the bones of several people and animals in the grave; a concealed niche in a nearby wall (not shown) contained the bones of a 60-year old man that had been removed from the grave.