The principal branch went underground to the cave reservoir at its mouth, on the level of the quay of the Marmorata between that and the Salaria, just outside of the old Porta Trigemina; the old _agger_ in which that gate was situated still forms the southern or lower boundary of the wharf of the Salaria. Part of the course of this earliest _specus_, that of the Appia, can be traced and seen in a subterranean stone quarry nearly under S. Sabba. The _specus_ is six feet high and two feet wide, and it is filled up to nearly half its height by solid clay, evidently the deposit left by the water. The old _specus_, long after it had been out of use, seems to have been employed by the quarrymen: as it was a tunnel in the tufa rock just high and wide enough for a man to walk in, by cutting away one side they made it wide enough for a horse and cart to carry the stone, and they raised the vault as high as was convenient for the purpose of making an entrance into the quarry. The series of wells descending into the old _specus_ from the gardens above remain at intervals, with notches for steps cut in the rock to enable a man to go up and down when required. The course of the _specus_ is cut off by the road to the Porta Ostiensis, but probably that road was originally carried over it[20] at the crossing; it then passed through another large subterranean quarry nearly under S. Prisca, to the cave reservoir at its mouth[21]. Before arriving at the two large reservoirs just outside of the garden of the Sessorium, now Sante Croce, the _specus_ of the Appia must have passed by another smaller reservoir at the same low level, near the ruins of the apse of a hall, miscalled the temple of Venus and Cupid. This seems likely to have been the point at which the branch _specus_, coming from the north, entered Rome, and it was then carried on to the two large reservoirs outside this garden, supposed to have been the _Gemelli_[22]. Below the “Salinæ” or salt warehouses on the bank of the Tiber, and near the “Porta Trigemina,” the water was “distributed.” This was also close under the Clivus Publicii, or the slanting zig-zag road leading up from the wharf to the top of the hill. So far the general course can be traced; but the exact point of entrance into Rome could not be fixed without excavations, which have not as yet been made. There are, however, some _data_ given by Frontinus which should not be overlooked, as they bear incidentally upon the course of some other of the aqueducts.