Sounds like Waterdeep and Undermountain.Mainly that it's located under a city, that everybody knows about it, and that it's a mainstay of the local economy. In fact, the Labyrinth is pretty much why the city exists — people come from all over to brave the depths of the maze in search of fame and fortune. Only the top three levels of the Labyrinth have actually been mapped, but I like that for the same reason that I like the 0One maps (because I can expand the Labyrinth as I see fit).