Absurdly Spacious Sewer/Analysis

Note that old cities of the classical era (like Rome and Athens) may have something approaching this, as the ancient Greeks, Romans, the Minoans of Crete, the Chinese, and Mayans all had the technology and know-how necessary for pipes and plumbing, built canal systems and aqueducts, and in some cases invented the water toilet, remains of which can still be seen today.

But many preindustrial "sewers" were essentially just very large holes in the ground meant to store, not transport, sewage, until the city contracted someone to break in and clean it out. In medieval European cities after the Dark Ages, the concept of "sewer" was reduced to a simple trench in the middle of the street; any waste would be thrown into the street, where rainwater would (eventually) wash it away. If the population were extremely lucky. And now you know why it's called The Dung Ages. The pre-Victorian London sewers, for instance, started out as rivers and creeks flowing into the Thames, which were later covered over as they became clogged with sewage washing in from the streets and draining from nearby cesspits. The old tunnels still exist and generally contain clean-ish river water now that the filth had been diverted to a more modern sewer system.

However, many ancient cities were built over the ruins of previous cities and had some tunnels beneath them. A few also had catacombs. Modern cities have storm drainage systems, some of which meet all the qualifications for this trope, although they aren't actually sewers, since their purpose is to drain rainwater and/or snowmelt from the city into the nearest body of water. For game purposes, these can be treated like Absurdly Spacious Sewers.