A long-forgotten feature of Ankh-Morpork, believed to have been built in the time of an earlier empire with higher standards of public cleanliness, as a major sewer for safely disposing of the city's various forms of waste. The project was ordered by - or at least built in the time of - Cirone IV, a King of Old Ankh.
As a couple of thousand years' worth of Ankh-Morpork was constructed on top of the Imperial one, with more free and easy attitudes to waste disposal, the Cloaca remained, but lost and forgotten. Until, during one summer heatwave when the level of the river was abnormally low, Lance-Constables Cuddy and Detritus of the Watch inadvertently re-discovered it.
The hole in the ground in Quarry Lane, that allowed them to escape from a mob of dwarfs and Trolls eager to smash their goolugog heads in for collaborating with the enemy, pitched them into a world of wonder and reminded them both of Home. Cuddy thought of Dwarf halls under the mountains; Detritus of the friendly caves of Home. They soon work out what it's for, however, and it plays a key role in the denouement of Men at Arms.
"Even der shit have a street of its own" Detritus murmured, in wonder. "Truly this is a city of riches and opportunity"
As Quarry Lane is in the rimwards-by-widdershins part of the city (to those thinking in Roundworld terms, the south-east), and they eventually resurface, after following the length of the Cloaca, in the University Library, at the other end of the City(turn-by-hubwise, or a long way north-west), then two things may be deduced.
- That the Cloaca runs the full diameter of the city, from bottom-right to top-left;
- That it is at such a depth that it runs underneath the River, as a direct line taken between Quarry Lane and the University crosses the River twice at the Isle of Gods. This would explain why it is only accessible in high summer or at time of drought: based on what we know on Roundworld about ancient drainage systems, this would have been deliberate, so that the River could be harnessed to flushing the Great Drain through. It is assumed that the Cloaca carries on past Quarry Lane and the city limits, to a point where it flushes directly into the Circle Sea. The text of Men at Arms confirms this:-
A very large tunnel, almost blocked with debris and silt, led off in what Cuddy thought was the direction of the estuary (p227, Corgi PB)
The assumption is also that lesser tributary sewers originating in other areas of the city periodically feed into the Cloaca, thus in theory at least, providing the main arteries of a sewerage network capable of serving the whole city's needs. Again, the text of Men at Arms confirms that the Cloaca is so arranged: Cuddy and Detritus initially land in such a tributary sewer, and follow it downwards into the main Cloaca. The body of someone that looked for all the world like Beano the clown is discovered concealed in one of many such lesser sewers, which all feed into the main drain.
tunnels radiated off it in every direction, and at various heights in the walls (p227)
No doubt their subsequent reports have been noted by those in positions of authority and the Cloaca may soon be open again, to do the job it was intended to do. Harry King is certainly keen to stake a claim on property in various, no doubt strategically important, areas of the City... given what he knows about the Undertaking, is it permissible to wonder where his business acumen is leading him?
There was even a goddess of the sewer systems of Rome (Cloacina, in case you're wondering who to pray to if your toilet won't flush). She must also be on the Discworld, as a Small God or at least existing in potentia right now?