The oldest monument of Roman masonry is the remaining portion of a cloaca in this district, commonly identified with the Cloaca Maxima of Livy, which reaches from a spot near the church of S. Giorgio in Velabro and the Janus Quadrifrons to the Tiber bank near the Ponte Rotto. The ancient archway has been broken open here, and can be reached by descending into a hollow near the Janus Quadrifrons. Near the Janus Quadrifrons, at the above-mentioned spot, seven cloacæ unite and pour their waters into the still extant portion of the Cloaca Maxima, so that a large stream is constantly flowing through it. These branch sewers are built with solid brick arches, but the main archway, though fronted with modern brickwork, consists of massive blocks of tufa, and at short intervals of every few yards has an arch of travertine introduced, to add to its solidity and strength. The original size of the archway, one-third of which is now choked up with mud, was twelve feet four inches high, and ten feet eight inches wide. Strabo and Pliny say that a cart loaded with hay could pass through some of the Roman sewers, and certainly in the case of this cloaca, it would not be impossible to do so were it cleared of mud. M. Agrippa, the Haussmann of Rome, is said, when ædile, to have traversed the main sewer in a boat. The whole length of this remaining portion is at least three hundred and forty yards, and it makes several bends, following probably the direction of the ancient streets. The mouth is still visible, when the Tiber is not high, at a spot called the Pulchrum Litus, near the round temple usually called the Temple of Vesta. The immense size is due to the fact that it was not only a sewer for refuse, but a drain for the lake of the Velabrum, and the many land springs of the Forum, and must be classed with the emissarium of the Alban Lake and other gigantic undertakings of the kind, such as the cuniculus at Veii, executed about B.C. 539. For a distance of about forty feet from the mouth the cloaca is constructed of a triple arch of peperino, mixed with some blocks of tufa, but throughout the rest of its course it consists of a single arch of tufa with occasional bands of travertine. The masonry along the embankment of the shore on each side, is partly of peperino and partly of tufa and travertine blocks laid along and across alternately.