By the time of the reign of Emperor Trajan (98 – 117), although 9 aqueducts were already in operation in the city, the area on the west side of the Tiber, the Trans Tiberimus, was still without drinking water. The only aqueduct reaching Rome from the northwest, the Aqua Alcietina, ran almost entirely underground except for a very short section.
Built solely to fill the water stadium of Emperor Octavian Augustus, this aqueduct carried water from the small lake Lacus Alsietinus (today’s Lake Martignano), completely unsuitable for drinking.
Trajan found clean springs in the same area, on the hills on the west and east sides of Lacus Sabatinus (modern Lake Bracciano) and the first section of the new aqueduct ran about halfway around the perimeter of the lake.
The emperor bought up all the land in the path of the future aqueduct, and his tunnel was able to pass completely through a protected area 30 feet wide from the lake all the way to Rome.
The entire structure was completed in 109. Most of it was at ground level, and only when approaching the city did the tunnel end up on top of the arched structure.
The aqueduct entered the city from the Janiculum Hill, at the top of which its main outlet was located. The steep descent from the summit to the Trastevere plain provided enough hydraulic power even for the local mills.
Trajan’s Aqua functioned until 537, when the Goths caused damage to the Roman aqueducts.
During the Middle Ages, it was attempted several times to be repaired, but never achieved stable operation until the very beginning of the 17th century, when the aqueduct was finally fully restored and even partially rebuilt.
It was renamed Aqua Paola, and stone pyramids were placed along the entire tunnel to mark the openings for repairs.
What can be seen today in the city limits are the three parts of the aqueduct belonging to its last mile before the main exit. They follow the Via Aurelia Antica, a modern road corresponding to the ancient Aurelian Road, still narrow in places and at times congested with traffic.
The aqueduct reaches its maximum height, about 6 meters, in the first part closest to the exit. This is not very high compared to other aqueducts. This section of the Aqua Traiana marks the boundary of the large public park of Villa Pamphili, the former country residence of the Pamphili family, whose most prominent representative was Pope Innocent X (1644-1655).
Where the Via Aurelia Antica makes a slight turn, the aqueduct crosses it through an arch that does not look antique at all. It was built by Pope Paul V in 1612 during the restoration of the Acqua Traiana, and is commemorated by a plaque and the pope’s family coat of arms at the top of the arch.
The text contains a curious error: by the third line, reading AB AVG CAES EXTRUCTOS (built by Emperor Augustus), it is clear that the pope and his architects mistakenly believed that they had rebuilt the Aqua Alcietina, built by Emperor Octavian Augustus, and not the Aqua Traiana. The same error is found in the inscription on the huge fountain of the Aqua Paola.
The aqueduct runs west along the right side of the road, getting lower and lower until the arches, suddenly, disappear altogether. Soon the road goes uphill a bit and then downhill again. Immediately afterward, the aqueduct appears again, already on the south side of the street.
This second part of the aqueduct, on the contrary, becomes higher and higher until it becomes again the boundary wall of Villa Pamphili. In fact, the main entrance to the park is located under one of the arches of the aqueduct.
The end of this part is barely recognizable, as the brick surface of the aqueduct is covered here with plaster and looks like an ordinary wall.
At the crossing, the Aqua Traiana breaks off again.
The third visible part of the Aqua Traiana is located 200 meters to the west, at the intersection with via del Casale di San Pio V, next door to a block of modern buildings.
The aqueduct is represented here by a series of arches, the upper part of which now protrudes from the ground to a height of 1 meter.
The full height of the structure in this place did not exceed 3.5 meters, i.e. it actually corresponded to the height of the water tunnel, which is 2 meters.
At the corner where the aqueduct bends, turning to the northwest, a preserved pyramid rests above the tunnel cover.
