Position: the remains of the Aqua Alexandrina

By a few minutes walk from Casa Vincenzo you can visit the remains of the Aqua Alexandrina. The Aqua Alexandrina was the last aqueduct built in ancient Rome, in AD 226, over 500 years after the first one. Its purpose, according to emperor Alexander Severus who was the sponsor, was to carry water to the Baths of Nero, which stood in the western part of the Campus Martis, more or less where today is piazza Sant'Eustachio.
The new aqueduct drew water from springs located east of Rome, as most previous ones did, and shortly before approaching the city walls it turned into an underground channel, and entered the urban area by Porta Praenestina (now Porta Maggiore); from here it likely followed the Esquiline hill, then the Quirinal hill, finally sloping down towards the plains of the Campus Martis.
None of its urban parts has been left standing, and the same aforesaid course is partially hypothetic because the underground duct has never been found. Instead, two long parts of the viaduct can be seen in the suburbs, while off the modern city's boundary the aqueduct is still in rather good condition for most of its length.
The first surviving part is located in viale dell'Acquedotto Alessandrino, where an unbroken series of arches stretches for 250 m (or yards) through a public garden. Their direction seems to be pointing towards via del Mandrione, not far from this spot. In fact, also the Aqua Alexandrina approached the city following the common route shared by the Aqua Claudia, Anio Novus, Aqua Marcia, Aqua Tepula and Aqua Iulia.
Not far from the south-eastern end of this part of viaduct, another interesting ancient remain can be seen in via Casilina: the monumental tomb of St.Helen, mother of emperor Constantine the Great, who died in 328 AD. It consists of a round hall built in bricks, with a tall dome (collapsed a long time ago), which stood by the entrance of a Christian underground cemetery, or catacomb, named after the saints Marcellinus and Peter.
The second standing part of the Aqua Alexandrina is about 1 km (0.65 miles) further east, at the southern end of another popular district, Centocelle. This stretch has more or less the same length as the previous one, but the brick texture of the pillars is better preserved. Luckily, the large and busy road that passes below its central part, viale Palmiro Togliatti, does not affect at all the surviving structure.
Towards both ends of this fragment the pillars become gradually lower, particularly on the left (i.e. towards Rome), where the modern ground level, much higher than in ancient times, soon reaches the top part of the structure, and completely buries it. Here the specus is quite poorly preserved: the only existing part is its floor, or bottom, which remains uncovered for a few metres or yards. But it is interesting to see it in section: the waterproof facing, almost 10 cm (4 in) thick, can be easily told from the rest of the texture made of mortar and bricks.