The sources of the Marrana are about a mile above Marino, and nearly three miles from Albano; they are in a long, narrow, deep valley in the rocks, and the water gushes out in several places at short intervals on both sides of this valley, which continues on under the rock on which the small town of Marino is built. Close under the town is a tall medieval tower, and at the foot of this is a _piscina_ of rude early character, the lower chamber of which is still full of water. A _specus_ cut in the rock as a tunnel is also visible at this point. On the side of the valley opposite to the town are the splendid ancient quarries of _peperino_, called by Vitruvius _lapis Albanus_, because Marino was in the district of Alba Longa. The stream flows on in the same deep valley, winding down the hill, and, at the foot of it, the Aqua Crabra, coming from Rocca di Papa, is united with it shortly before it is crossed by the bridge on the road to Grotta Ferrata, ten miles from Rome and near the tunnel through which flows part of the united water, the main body going on straight to the river Anio; the part which is carried through the tunnel turns at a sharp angle to the left, or west, towards Rome. At each end of the tunnel is a loch of early character, partly cut in the rock and partly built of travertine, with the grooves of flood-gates. Over the exit from the tunnel is a piece of old wall faced with _Opus Reticulatum_, of the same rude early character as that of the Aqua Julia. After the water emerges from the tunnel, it passes in a deep bed to a bridge over the stream of the Marrana, near the _piscinæ_, six miles from Rome. At this point the deep foss of the river Almo is close to it. This is dry in dry weather, but has abundance of water in wet weather, and here the foss is dammed across; this appears to have been done originally to effect a junction with the water of the Marrana, which from this point flows in the foss of one of the many branches of the Almo that intersect the Campagna in this part. This mountain-stream is a very uncertain one; sometimes the deep foss is full of water, flowing at the rate of five or six miles an hour, at other times it is dry. Numerous winding streams are collected into one at the head of the valley of the Caffarella, near the church of S. Urbano. Here it passes through swampy ground at a place called Aqua Santa, on the cross-road from the Via Appia Nova to the Via Appia Antiqua; here also several springs fall into the deep bed, one of which is miscalled the fountain of Egeria[193]. In dry weather this branch of the river Almo appears to begin here, and from thence to the mouth of this branch near S. Paul’s, mentioned by S. Gregory as the Almo, the water never fails. All the other branches are deep, dry fosses after dry weather, and the branch that now flows through Rome must originally have been frequently dry, and at other times liable to floods[194].