Sewer

The standard adventure setting with the most historical and realism baggage.

Existence in various settings

One important fact: A lot of historical cities lacked sewers. This lead to problems with fecal-spread disease, obviously, but people at the time didn't know that. All they knew was that shit stank, and getting rid of it further then out on the street was a luxury, particularly when horses were also a common mode of transport.

Which is not to say they didn't exist, it's just that they were rare.

Modern sewers, on the other hand, are actually rather tiny. A typical sewage pipe maxes out at 12 inches. And, once the pipe is in, you rarely have a need to access it, except to clear it out, which is more easily done with various tools that go inside the pipe, or replacing it.

On the other hand, there were frequently tunnels beneath the ground, particularly in cities prone to siege or in places with tombs. Further, flood drainage tunnels (where rainwater goes to prevent flooding) have always been a thing. But neither of these are sewers.

Realism

Feces, in sufficient quantities, starts emitting various noxious gasses that make it annoying to be around: The partially digested food attracts bacteria that emit methane (which is somewhat poisonous to oxygen breathers), and the partially digested food is probably stinking up the joint all on its own.

So, being in a sewer is not a good idea.

Our fiends at TvTropes have an article about this under Absurdly Spacious Sewer. Scrolls down to the real life section for a good list of sewers and not-quite sewers that are actually real.

Sewer Analogues

For the purposes of adventuring beneath a populated city into a forgotten, semi-dangerous part that nobody wants to go, a more appropriate location would be a catacomb. Because burial space is at a premium in densely populated cities, catacombs make a good if spooky alternative. The old catacombs of Paris and Rome were quite large and full of maze-like corridors, allowing for all sorts of secret meetings and illegal enterprises for the criminal underworld. Also, all the bones and skulls stacked everywhere provides for the festering ambience that a sewer would provide.

Notable /tg/ Sewers

• Just about any Thieves' Guild is probably going to be connected to one--although, when actually shown, the result is usually more "Flood tunnel" than "sewer".
• Warhammer Fantasy's Empire had sewers with Skaven, Chaos Cultists, and mutants hanging around, among worse things.
• World of Darkness/Chronicles of Darkness loves them, particularly for Vampire lairs.