Undertown is a vast network of abandoned tunnels and sunken streets underneath the city of Chicago.
Undertown is a network of caves, ruined buildings, tunnels, and ancient construction underneath Chicago. It was a dark, damp, cold place full of various creatures that shunned sunlight and human company. It might have been radioactive. 
In Dresden-Verse, Chicago was a swamp, level with Lake Michigan. It kept sinking and the City kept building and layering upwards. The lower levels exist as a whole city under the street level of the City proper. There used to be a problem with rats and criminals there that got crowded out by supernatural things that don't take well to sunlight, creating a mini-civilization.
In the Series Edit
In Summer Knight, it served as the entrance for the Winter Court . When Harry went to talk with Maeve, the Winter Lady, he was guided through Undertown by Elidee of the Wee Folk, one of Toot-toot's people, entering it through the subway at Randolph and Walbash. He was met by
In Death Masks, it is where the Denarians had setting up shop. Nicodemus Archleone held Harry captive under running cold water to nullify his magic. Nico tried to rcruit Harry into the Denarians. Shiro traded himself for Harrry and then gave his Sword, Fidelacchius, into his keeping.
- Billy: "So you're taking us into a maze of lightless, rotting, precarious tunnels full of evil faeries and monsters."
- Harry: 'Maybe leftover radiation, too."
- Billy: "God, you're a fun guy." 
- ↑ 1.0 1.1 Death Masks, ch. 21
- ↑ 2.0 2.1 2.2 2.3 Death Masks, ch. 14
- ↑ Death Masks, ch. 22
- ↑ White Night, ch. 15
- Little Chicago
- List of Faerie characters
- Winter Lady
- Billy Borden
- Little Chicago
- Vittorio Malvora
- Nicodemus Archleone
- Creatures - Dresden Files
- Google Maps - interactive picture of corner in daylight—arrows on bottom move your view around intersection, also clicking cursor on places, if get too close zoom out a little
- Randolph and Wabash | Flickr - Photo Sharing!
- Randolph and Wabash - II | Flickr - Photo Sharing!
- Randolph and Wabash - III | Flickr - Photo Sharing!
- Google Image Result for http://farm9.staticflickr.com/8249/8458803798_388e3ed984_z.jpg
- Map of Wabash Randolph in Chicago
- Randolph/Wabash Station - CTA 'L' Train Station Information