The Aqua Aurelia of the Regionary Catalogue must be the one made by Marcus Aurelius to convey water to his villa on the Via Appia, usually called the Villa de’ Quintilii, where a brick-stamp of the time of Marcus Aurelius (A.D. 162) was found on the aqueduct itself by Fea in the eighteenth century[181]. This water was afterwards conveyed to Rome by his successors, Ælius Aurelius Commodus and Septimius Severus, to supply the great _thermæ_ begun by Commodus and finished by Septimius Severus, which were in the first Regio just inside of the Porta Latina, and must have been begun about A.D. 185[182]. The Aurelia was an important aqueduct, and there are slight remains of its _piscina_ just outside of the Porta Latina on the southern side: its _specus_ was traced nearly to that point in the excavations made in 1871. The modern road is cut through this old _piscina_, forming a foss-way in this portion: the aqueduct there passes underground, and has been destroyed or is not visible in this part; but, about a mile from Rome, it can be seen on the bank in the western cliff of the valley of the Caffarella. The _specus_ remains as a tunnel for a short distance, and this was laid open at both ends in 1872 under my direction; it had been concealed and covered over with Pozzolana sand, which had fallen over it from the cliff above, but it can now be seen again. The greater part of the _specus_ has been destroyed, and used for building the large modern farm-house on the opposite side of the valley, the building material having been good brick. As it would not pay to destroy the tunnel, that was let alone. These remains of the _specus_ are near the well-known tomb called Dio Ridicolo. To this point it runs along the bottom of the valley at the foot of the hill on the side of the small stream or Marrana, here artificial, and parallel to that branch of the river Almo. This open conduit was probably made in the twelfth century, at the same time that the other branch of the Almo, which runs through Rome, was altered and made into a canal or mill-stream where necessary, in order to keep up a constant supply of water, and to avoid its being wasted in floods.