Cloaca Maxima: Ancient Rome's Great Sewer and Its History

The Cloaca Maxima, the “greatest sewer”, was the main drain of ancient Rome, carrying water from the valleys around the Forum into the river Tiber. Roman tradition credited it to the city’s kings in the 6th century BC. Repeatedly rebuilt and extended, it made the marshy heart of Rome habitable, and parts of it still function today.

A city built on wet ground

Rome’s hills rise from low valleys that were once boggy and prone to flooding. A stream ran from the area of the Subura, between the Esquiline, Viminal and Quirinal Hills, down through the valley of the future Forum and the Velabrum to the Tiber. When the river rose, the low ground could be under water.

For the hilltop villages of early Rome to share a common centre, this water had to be controlled. In the 7th century BC the Forum valley was raised with fill and given its first paving, and the stream was channelled. This canalised watercourse was the beginning of the Cloaca Maxima, and it made possible the Roman Forum as a public square.

The royal tradition

The historian Livy, writing in the late 1st century BC, attributes the drainage of the Forum to Tarquinius Priscus, Rome’s fifth king. He credits the great sewer itself to the last king, Tarquinius Superbus, who reputedly forced the common people to dig it as well as build the Temple of Jupiter on the Capitol. In Livy’s telling, this harsh forced labour was one of the grievances that led to the end of the monarchy and the founding of the Roman Republic.

Pliny the Elder, writing in the 1st century AD, was amazed that the sewers had survived some 700 years of floods, earthquakes and collapsing buildings. He described the underground channels as large enough for a wagon loaded with hay to pass, a claim also made by the geographer Strabo.

What archaeology shows

Modern study suggests that the early drain was largely an open channel lined with stone, not the covered tunnel of popular imagination. Some early walls in grey volcanic tuff may date from the 6th century BC, which fits the tradition in broad terms.

The channel was enclosed and vaulted in stages. Much of the surviving masonry, including stretches of fitted stone blocks and the arched outlet on the Tiber, is thought to date from the later Republic, probably from the 2nd or 1st century BC, with further repairs and brick additions under the emperors. Recent studies, including work by the architectural historian John Hopkins, have examined its course and building phases in detail.

As Rome grew, side drains from public baths, latrines, streets and private houses fed into the main channel. The Cloaca Maxima thus became the trunk of a wider network of sewers that removed rainwater and waste from the city centre.

Agrippa’s restoration

In 33 BC Marcus Vipsanius Agrippa, the close ally of the future emperor Augustus, served as aedile, the magistrate responsible for public works. He undertook a major overhaul of Rome’s water supply and sewers. The historian Cassius Dio records that Agrippa sailed through the sewers in a boat to inspect them, an image that has become part of the drain’s fame.

Agrippa’s work was part of a broader programme that also included new aqueducts. Clean water flowing in through the aqueducts of ancient Rome and dirty water flowing out through the sewers worked together as a single system.

Religion and later history

The Romans associated the drain with a goddess, Cloacina, who was later merged with Venus as Venus Cloacina. Her small round shrine stood in the Forum near the Basilica Aemilia, at the point where the drain crossed the square. The cult links the sewer with ideas of purification as much as with plumbing.

After antiquity the Cloaca Maxima was never wholly abandoned. Its outlet, a semicircular arch in the Tiber bank near the Ponte Palatino and the remains of the ancient Pons Aemilius, attracted artists and travellers from the Renaissance onwards, including the Danish painter C. W. Eckersberg in 1814. When the Tiber was walled in with high embankments from the 1870s, the outlet was preserved in the new wall.

Timeline

• 7th century BC: The Forum valley is drained and the stream through it is channelled.
• 6th century BC: Tradition credits Tarquinius Priscus and Tarquinius Superbus with major drainage works.
• 2nd–1st century BC: The channel is extensively rebuilt and vaulted in stone.
• 33 BC: Agrippa, as aedile, restores the sewers and reputedly inspects them by boat.
• 1st century AD: Pliny the Elder praises the sewers’ durability.
• 1814: Eckersberg paints the outlet on the Tiber.
• 1870s onwards: The outlet is incorporated into the new Tiber embankments.

Why it matters

The Cloaca Maxima shows that early Rome’s rise depended on engineering as much as on conquest: without drainage, there would have been no Forum. Still partly working after more than two millennia, it is one of the oldest functioning pieces of urban infrastructure in the world.