Cloaca Maxima: the stream that drains all the valleys between the Quirinal and Esquiline hills, running through the middle of the Forum Romanum on a northeast/southwest course and emptying into the Tiber a little below the east end of the island. It became one of the main drains of Rome, and Pliny (Pliny HN 36.105) says it gathered into its course seven tributaries. There is no reason to doubt this, and all may have been more or less permanent waters. Livy (Livy 1.38.6, Livy 1.56.2) says the work of making the brook a sewer was carried out under Tarquinius Priscus and Tarquinius Superbus, the latter using forced labor of the plebs, but he must mean the channeling of the course, because in the time of Plautus (Plautus Curc. 476) there was still an open channel in the middle of the Forum Romanum.

It follows a very irregular course, reflecting the Roman's reluctance to interfere with a potentially hostile and dangerous power. Within the excavations in the vicinity of the forum, it can be followed the length of the Forum Nervae and clearly continued beyond, culverted under the Argiletum (Fig. 39). At the lower end of the Forum Nervae it divides; one channel, the Braccio Morto, runs beneath the Basilica Paulli, the other, probably an artificial modification, runs around the end of the basilica to rejoin the Braccio Morto at the little sacellum of Cloacina. On the south side of the Forum Romanum, it runs under the Basilica Iulia parallel to the Vicus Tuscus, presumably having run in an open channel earlier, and zigzags through the Velabrum, running under the Ianus Quadrifrons and between the round and rectangular temples ("Vesta" and "Fortuna Virilis") of the Forum Boarium. Its mouth on the Tiber, framed in three concentric arches of Gabine stone, is a conspicuous landmark kept visible in the modern Tiber embankment.

Except for the Braccio Morto, the walls of part of which are of cappellaccio, the lower course has walls of squared blocks of Gabine stone, a floor of selce paving, and a vaulted roof of concrete and brickfaced concrete. The Gabine stone walls must be due to the general rehandling of the water and sewer systems of Rome by Agrippa beginning in his aedileship in 33 B. C. (Cass. Dio 49.43); Gabine stone was relatively expensive and used extensively only from the time of Julius Caesar through that of Nero (Blake 1947, 38-39), and Narducci describes the construction as very similar to that of the lower course of the Petronia Amnis (q.v.) from Piazza Mattei to the Tiber (Narducci 36-37, 40-42). The rest of the course is presumably of later date, reworked as new building made demands for more capacious branch lines. Ultimately, it received the wastes from the Thermae Diocletianae.

The Cloaca Maxima was an object of great admiration to Pliny for its size, so great that men could traverse it in boats, and so strong that the walls could resist the most violent storms, and even floods when the water backed up in its channel, and the roofs stood up under the passage of great blocks of stone in the streets overhead and the collapse of burning buildings (Pliny, HN 36.104-8). It continues to function today, but connected with the main sewer of Rome to prevent the backwash from flooding the Forum. Ficoroni (Le vestigia e rarità di Roma antica [Rome 1744], 1.74) reports that the lower course was all cleared in 1742, the conduit being found 10 m below the ground level and built of blocks of travertine. Narducci (4) gives the dimensions at the Tiber mouth as 4.50 m wide and 3.30 m high, while at the corner of Via di S. Teodoro and Via dei Foraggi it is 2.12 m wide and 2.72 m high. PA gives the dimensions in the stretch from the northwest corner of the Forum Augustum to Via Alessandrina as 3.20 m wide and 4.20 m high. Despite the variation, it was clearly capacious through the area of the Subura (cf. Strabo 5.3.8 [235]).

Nash 1.258-61; Roma sotterranea, 170-72 (C. Mocchegiani Carpano).