This aqueduct was made in the year B.C. 145, and has its source a few miles below Subiaco; the springs are collected in a small lake called Aqua Serena (1537), the water from which flows into the river Anio; but a portion of it was intercepted for this aqueduct, and is now again taken from the same spot and brought into Rome. It has always been celebrated for its extreme coldness and great purity. The old _specus_ or viaduct was found in 1870 in this lake, having long been concealed by being under water; but by drawing off some of the water it was brought to light, and the engineer of the new company decided on carrying his new _specus_ up to this point. The previous plan had been to draw the water from another lake nearer to Rome, which is nearly equally good water, but not the real Aqua Marcia so much prized, and this is now again brought into Rome. The source of the old aqueduct is celebrated for its extremely picturesque character, as well as for the fine construction of the arcade of large square stones, and the scientific arrangement of the reservoirs (_castella aquarum_) and filtering-places (_piscinæ_). The sources in the Aqua Serena are shewn in Nos. 1538, 1539. Another source for an additional supply was on the lake of the Mole d’Agosta, 1543. In the earlier part of its course the _specus_ is underground; but on arriving at the valley called the Valley of the Arches, about two miles above Tivoli, it emerges upon the fine arches which give the name to the valley, and is here carried across the river Anio. These arches are in two series, and are among the most picturesque objects in the neighbourhood of Rome; the effect is improved by a medieval tower built upon the first pier of the bridge. This is shewn in Nos. 1053, 1054. It then appears again on the other side of Tivoli, on the road called the Promenade of Garciano, which is on the edge of the hill looking towards Rome, and above the winding road up the hill; S. Peter’s is visible from this point. On this platform there are a number of fine remains of the _specus_ and of the _castella_ of the Aqueducts, some of the finest of which are of the Marcia. The Plan and Section of a very fine Reservoir and Piscina here is given in No. 535*. Another very fine one has a wall on the edge of the cliff of the character called Cyclopean (Nos. 1513 and 1528). Chambers of this remarkable early reservoir are shewn in Nos. 1520 and 1521. A considerable part of this aqueduct was rebuilt by Augustus[224] (B.C. 11), and again about a century afterwards by Trajan[225], and of the _specus_ of that time we have examples in 1524 and 1525, shewing a very peculiar kind of the ornamental construction called _Opus Reticulatum_. Another reservoir of this time is shewn in 1526, and an earlier one belonging to the original construction of the kind called _Opus Incertum_, which probably belongs to the earlier period, is shewn in 1527.