The “Under” is the colloquial word for the underground tunnels that sprawl underneath the surface of the city. Anyone who is anyone has business here, whether they like it or not. Even though the city looks beautiful on the surface, the Under reveals its real face.
There is no telling how deep the tunnels go, but speculation goes from hundreds of feet to dozens of miles.
Near the top the tunnels are ubiquitous, so it is more of a porous rock than individual tunnels. Deeper down the tunnels start to be fewer and they cross less, making it harder for people to get from one place to another. There is always digging going on everywhere in the Under, mostly on the fringes where the rock is still unexplored, but many passageways are dug between tunnels or homes.