[21] In the spring of the year 1870, another excavation was made close to this point, and a way was found into another old subterranean stone-quarry long out of use. Through this cave, or quarry, the _specus_ of five different aqueducts pass on their way to the Tiber. Some of these come down at a steep decline, and the water of the whole seems to have been carried into the lowest one, the Appia, at this point. This _specus_ must have been carried over the deep foss-way upon or under the arch of the gate of the old wall of _the city_, where four roads meet. It is also visible again in another old subterranean stone-quarry on the other side of the road, nearly under S. Prisca, and from thence it must have gone to the old cave, used as a reservoir near the Marmorata, and the Porta Trigemina, immediately under the monastery of S. Maria del Trinita di Malta, where the _specus_ is again visible, and where the wells of other aqueducts run into the same cave reservoir at the mouth of the aqueducts in this part of Rome. One of these runs down a vertical pipe from the reservoir nearly over this cave, but under S. Sabina on the hill above, excavated in 1865, and described by M. Descemet (Sect. xi.) There is another large reservoir in the interior of the hill, still full of water, supplied by a spring rising there; the water from this still passes through the same passage to the Tiber. This is also said to have been called the cave of Faunus by the poets. It is probably also the same as that of Cacus, being a large natural cave, with a spring of water, and a natural reservoir of considerable size in it about knee-deep, the entrance to which is by a narrow passage made into the _specus_ of the aqueduct. Such a cave might very well have been used to drive cattle into for concealment, and a resolute, well-armed man standing at the entrance might defend it against any number. Solinus (i. 7) says that the cave of Cacus was at the Porta Trigemina, and that he dwelt in the Salinæ, which are close by this spot. “Qui Cacus habitavit locum cui Salinæ nomen est, ubi Trigemina porta.” [22] On the wall of the smaller reservoir, the fragment of an inscription, relating to the Thermæ of S. Helena, now in the Vatican Museum, is said to have been found:— D. N. HELENA ... VEN. ... AVG. MAT. AVIA ... BEATIS ... THERMA ... SI ... ESTRV ... [23] Frontinus says, c. 5, at the sixth milestone on the Via Prænestina, about nine hundred and eighty paces off to the left, and near the Via Collatina, this stream has its source. The sources both of the Aqua Appia and of the Augusta were traced by Signor Fabio Gori and Mr. J. H. Parker, in March, 1868, and were afterwards shewn to the British Archæological Society of Rome.