Undertown

Undertown is a vast network of abandoned tunnels and sunken streets underneath the city of Chicago.

Description
Undertown is a network of caves, ruined buildings, tunnels, and ancient construction underneath Chicago. It is a dark, damp, cold place full of creatures that shun sunlight and human company. It might be radioactive.

Goblins, spirits of the earth, wyrms, vampires, trolls, molds and fungi not of the natural world, and things wizards don't even have names for, come from here.

History
Dresden's 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 that got crowded out by supernatural things that do not take well to sunlight, creating a mini-civilization.

Harry claims that it once housed the Manhattan Project. The tunnels that once housed it were just the start of Undertown.

Summer Knight
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. They entered it through the subway at Randolph and Wabash.

Death Masks
In Death Masks, it is where the Denarians had set up shop. Nicodemus Archleone holds Harry Dresden captive under running cold water to nullify his magic. Nicodemus tries to recruit Dresden into the Denarians. Shiro trades himself for Dresden and then gives his Sword, Fidelacchius, into his keeping.

White Night
In White Night, it was where Vittorio Malvora met with Cowl while Harry Dresden spied on them using Little Chicago.

"Something Borrowed"
In "Something Borrowed", Georgia is kidnapped by Jenny Greenteeth, put under a sleeping spell, and brought to Undertown, where she was guarded by shellycobbs. Dresden and Murphy are forced to fight them in order to rescue her.

"Heorot"
In "Heorot", Harry Dresden and Ms. Gard raid undertown in the search for Elizabeth Braddock.