The Cloaca Maxima was the great sewer of ancient Rome, built by the third-to-the-last king of Rome, Tarquinius Priscus, around 600 BC. It was built to drain the swampy land of the Roman Forum. Originally several streams met not far from the east end of the forum and flowed down the valley to empty into the Tiber River. The construction of the Cloaca Maxima (literally, 'Greatest Sewer') took the stream underground and through an arch at the river which can still be seen near the modern Ponte Palatino (1st and 2nd photos below, and closeup in the 3rd photo below). Just walk down the steps to the river level from the city-center side of that bridge to get a close-up view of the sewer's exit point. Being an underground sewer, it is hard to get a good look at it; you can see a drain for it in the Shrine of Venus Cloacina in the Roman Forum, and where it ran through a back yard near the Arch of Janus, and where it drains into the Tiber River, all during daytime hours only.

    

The exit of the Cloaca Maxima into the Tiber River is hiding in the bottom-center of this arch
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The exit of the Cloaca Maxima into the Tiber River. A homeless family sleeps next to the drain.
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The exit of the Cloaca Maxima into the Tiber River
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Etruscan engineers planned the Cloaca Maxima, and semi-forced labor from the poorer classes of Roman citizens built it. The construction of the sewer spanned hundreds of years: initially it was an open drain; over time it was deepened and built over as city land became more valuable. The continuous flow of water into the city from the aqueducts helped remove waste from the sewer and keep it clear of obstruction. There were many branches off the main sewer, but those were all for public sites like public baths, public toilets and public buildings. Private residences did not connect to the sewer and probably had cess-pits. Agrippa overhauled the sewer during Augustus' reign in 33 BC, and building style and material evidence suggests it was regularly improved. Even today, the Cloaca Maxima drains rainwater and debris from the center of Rome, below the ancient Forum, Velabro and Forum Boarium. Other than the drain into the Tiber River, the other places you can see the Cloaca Maxima are in a private back yard (look through the gate) across Via del Velabro from the Church of San Giorgio al Velabro (1st photo below), at the Shrine of Venus Cloacina in the Roman Forum (2nd photo below), and at the eastern stairs of the Basilica Julia in the Roman Forum, where a door leads to the sewer. The Arch of Janus is erected right on top of the Cloaca Maxima. The Mouth of Truth might have started its life as a manhole cover for the Cloaca Maxima before it moved on to become a lie detector.

    

A fragment of the Cloaca Maxima, beside a house across the street from the church of San Giorgio al Velabro
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Shrine of Venus Cloacina, in the Roman Forum, the spot where the Cloaca Maxima enters the Forum
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• My Cloaca Maxima photos
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