Under the street level is the Underworld. The Underworld is the part of the city where energy, waste, nourishment and information from the Upperworld is collected, refined and reused. If the Upperworld is hard to orient in, the Underworld is a nightmare. Inside great pipes the waste of millions are transported to structures turning it into nourishment for the City and its inhabitants. Tunnels connect chambers holding unknown devices and structures.
Great halls stand empty, flanked by immovable statues.
Dead parks wither in sunless darkness. Whole buildings seems to have sunk down into the Underworld and become part of it. Deeper down, the tunnels become less and less traversable for humans, filled with detritus, strange growths and hinders. Nobody has ever seen the roots of the City. The Monuments are strange.