When you live at the bottom, the only thing you have is to look up. If you're lucky you can see the lights of the great towers and the stars. If you're like most people, you just look at concrete. But the stars and lights are still up there.
These well crafted Dwarventi stairs lead down into one of the three main passages of Undertown. The underfolk have cleverly made it look like a short surface streets. Some of the branching passages look like streets as well, but it is all just a facade, soon they become more mine or hall like. The streets are “lit” (dimly by any topsider’s opinion) by mage lights and a few ancient amber Dwarventi Lights.
For all the aquaducts coming into the city, there munt be a place for the water to leave. Thus, many years ago, the Blackwater Tunnels were commissioned to carry sewage away from the city buildings.
Teenagers who run away from home to live in the wilderness may become feral. As they become more and more feral, they gain some supernatural powers (a la Peter Pan - ability to fly, things they imagine come true - possibly quite powerful), but at the expense of their humanity and ability to speak, comprehend complex subjects, etc. Maybe they have a leader: the most feral of them all.
A regression into a fantasy childlike state.