Location from The Dresden Files by Jim Butcher
The vast tunnel network beneath Chicago — older than the Great Fire, deeper than the subway, and inhabited by things that prefer the dark.
Undertown is what happens when a major city is built, destroyed, and rebuilt on top of itself for a hundred and fifty years. After the Great Chicago Fire of 1871, the city didn't excavate its ruins — it built over them. After the reversal of the Chicago River, whole sections of infrastructure were abandoned underground. The result is miles of tunnels, basements, sub-basements, forgotten utility corridors, and chambers that no city map acknowledges, stretching beneath Chicago like a second city made of darkness. The supernatural community knows Undertown intimately. Vampires — particularly the Black Court and the Fomor — use the tunnels for travel and shelter. Ghouls nest in the deeper chambers. Things that don't have names in English live in passages that haven't seen light since before the city existed above them. The tunnels connect to the Nevernever in places where the boundary between worlds is thin — weak spots where a sufficiently motivated being can cross between the mortal world and the spirit world without a proper Way. Harry has fought running battles through Undertown. He's also used it for travel when surface routes were compromised. The tunnels are not mapped comprehensively — every time someone thinks they've charted a section, they find new passages, or old passages have moved, or something has carved new ones. Going into Undertown alone is inadvisable. Going into Undertown without magical defenses is suicidal. Going into Undertown at night is both.
Tunnels, storm drains, abandoned subway passages, and things older than the city built on top. Dark, wet, cold. The deeper you go, the more the walls stop being concrete and start being rough-carved stone. Things live down here that haven't seen sunlight in centuries.
Also known as: Undertown, the tunnels, beneath Chicago