The caves in the woods
Actually, there are some archaeological records which may help. It deals with cavities dug in the natural tuff bastion delimiting the compound of St. Peter to the north.
The first, in the west, was simply called “the cellar of friars” – actually, in the profile of the entrance with a round arch – and by carving inside, it shows affinities with the Etruscan tombs in Viterbo.
The second is undoubtedly an Etruscan columbarium provided with recesses for depositing the ashes of the deceased. It is half filled with buildup of soil due to natural events – landslides, perhaps – and is very similar to others found in the Massa area, in Busseto and Castel Rinaldi.
The third cave in the east has an entrance and a vault suggesting to be a mithraeum, proved also by some traces of a wall sculpture where one could recognize the cosmic cloak of the god Mithras (the entrance in the shape of “L” and the dome-shaped ceiling had to give the impression of being in the center of the earth, in contact with the forces of nature).
During the Roman age, the mithraeums were places devoted to worship the god Mithras and imported into Italy after the expansion of the Roman Empire in Middle Eastern Countries. It was a Syrian cult, very similar to Christianity – Mithras was born on December 25th.
These three cavities have not been investigated by archaeologists, but only by the writer who is deeply convinced of their identity.