At this second milestone also is another _castellum aquæ_[39], mentioned by Frontinus as two miles from Rome. This is near the Porta Furba; it is entirely buried, but the vault of it is not many feet underground. In the spring of 1871, some excavations were made under my direction in a large vineyard hard by, and another subterranean reservoir was found near the road to Tusculum and Frascati, with a _specus_ cut in the rock going in the direction of that road, and apparently passing under it, on the line of a cross-way to the Via Appia Nova, a short distance only. On the other side of that is the “Albergo dei Spiriti,” near the junction with the Via Latina; and, in the garden at the back of that house, a _specus_ was found in a stone quarry, the vault of which had fallen in and brought the _specus_ to view. It seems to have passed underground along the southern side of that ancient Via for a short distance, and then crossed it to a _piscina_, of which there are remains at the foot of the bank on which the road runs in that part. It then goes along the edge of some higher ground, and for a short distance underground again towards a by-lane (_diverticulum_), parallel to the Via Latina; remains of a brick arcade can be seen on the bank of that lane, which is a deep foss-way, and goes on to Rome about a mile distant. It is cut by the railway before it arrives at the wall of Rome. It then passes through that wall and underground again as far as the arch of Drusus, over which it passes; and thence on an arcade, part of which only remains, to the great _piscinæ_ at the back of the _thermæ_ of Caracalla[40].