From the great subterranean reservoir, still in use for the water of a spring, near the Arch of Dolabella, to the edge of the western cliff of the Cœlian is but a short distance, and here the _specus_ seems to run in a bank dividing the vineyard or garden of the Villa Mattei from that of the monks of S. Gregory, on which a wall has been built. At this point subsequent alterations, and especially those under the Emperors Nerva and Trajan, have buried to a great depth the actual remains of the original aqueduct; but this later work occupies the same line, and must be described. We first come to a large _piscina_ or filtering-place on the cliff of the Cœlian, immediately above the Porta Capena. This _piscina_ is divided into two parts, one above the other; but both are below the top of the cliff, and are faced with brick and reticulated work of the time of Trajan. There are remains of two _specus_ running along against the face of the cliff; of the upper one the lower part, or pavement, of _Opus Signinum_, only remains, and this runs down in a sloping direction from the upper reservoir to a lower one, a little to the south of it, which is very extensive. This lower reservoir consists of several parallel chambers, through which, or rather in front of which, another _specus_ runs at a lower level; this appears to be horizontal. This _specus_ goes on in the direction of the ruins of a building of various periods, possibly the remains of the Ædes Camenarum, and passes then to the south in the direction of the Thermæ of Severus and Commodus.