D&D 5E - How do you make sewer dungeons believable?

This is a silly setting problem to have, but it's recently come up in my megadungeon game. Or more specifically, in the small town beside the megadugenon. How do you make sewer dungeons believable?

To the best of my knowledge, medieval sewers were little more than open air ditches. Rome's Cloaca Maxima is a good model, but it's the sewage system of a world capital. I doubt that my little dungeon-adjacent village could justify such an engineering project. So when you're in a village setting rather than a major city, how do you justify sewer dungeons? Or are you better off inventing some other kind of subterranean labyrinth and saving yourself the verisimilitude hassle?

(Comic for illustrative purposes.)