Analysis / Absurdly Spacious Sewer

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.
This trope is ultimately innaccurate. Even modern sewers can have impressively spacious and large tunnels and pathways. Many are to allow high volumes of material and easier maintenance access.

Plenty of old sewers are also fairly spacious for the same reason.

There is a large variety of sewers, sewer lines, and general sewage handling.

It should be noted there is a difference between a sewer and a sewer pipe. The pipes are what tend to be small.

As for creatures living in them. It happens more then a few animals are hardy enough to survive in the sewer.

A simple image search shows sewers ranging from small to massive cavernous underground structures across the world.

~Tuefel Hunden IV
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