There can be no doubt that this part of the building is of great antiquity. But the proofs that it ever formed a prison are not so clear. This has been inferred from the striking account of the imprisonment of Jugurtha by Sallust, who states that Jugurtha was placed in a cell with water at the bottom, and exclaimed, “Hercules, how cold your bath is!” Hence it has been thought that the prison must have had water in it, as this chamber has on the floor. Another proof that this is the ancient prison, called in the middle ages the Mamertine prison, from a statue of Mars or Mamers, or from the Forum Martis near it, has been derived from the statements of many Roman authors, who place the prison on the slope of the Capitol, near the Forum, and speak of an inferior as well as a superior chamber. The prison was probably in this neighbourhood, but the shape of the conical vault is rather that of a well-house. Mommsen has therefore suggested that this was the original purpose of the lower chamber, and that it was used as a cistern for collecting the water from the surrounding slopes. The top of the ancient conical vault is truncated and closed, with the exception of a round hole, by slabs of stone fastened together with iron cramps. A communication with the arched sewers which run down to the Forum, and also with an archway which reaches up the slope to some large chambers on the north-west under the Vicolo del Ghettarello, has been opened by Mr. Parker, and these longer vaulted chambers which are of great antiquity, have been taken to be extensions of the original regal prison. But there is no sufficient evidence to show that these arched passages were used for any purpose of transit, and they were more probably channels for draining off the water, which would otherwise have accumulated in the chambers or on the slopes.[64] An inscription is fixed in the outer wall, recording a restoration of the building by the Consuls C. Vibius Rufinus and M. Cocceius Nerva, as ordered by a decree of the Senate. These two men were consules suffecti before A.D. 24, probably in A.D. 22, the ninth year of Tiberius. But the name of the building is not mentioned in this inscription, and it seems uncertain whether it has not been removed from elsewhere. There was another and larger prison in this district, and this larger prison was called the Lautumiæ. Forty-three Ætolian prisoners are said by Livy to have been crucified in the Lautumiæ, which must therefore have been certainly much more extensive than the cell which is called the Carcer Mamertinus. Seneca also mentions the request of a prisoner, Julius Sabinus, that he might be removed from the Carcer to the Lautumiæ.[65] There seems, therefore, to be no distinct proof that the conical cell was ever anything more than a water tank, and the name of prison which has been attached to it is probably a mediæval legend invented to indicate a spot which might be venerated as the prison of St. Peter.