Cuts open fence

THE RAIN WAS WEAKENING THE TRACKING SPELL ON my amulet and washing away both the scent of the grendelkin and the psychic trail left by the terrified Elizabeth, but between me and Mouse we managed to find where the bad guy had, literally, gone to earth. The trail ended at an old storm-cellar-style door in back of the buildings on the east side of Wrigley Field, under the tracks of the El, near Addison Station. The doors were ancient and looked like they were rusted shut—though they couldn't have been, if the trail went through them. They were surrounded by a gateless metal fence. A sign on the fence declared the area dangerous and to keep out—you know, the usual sound advice that thrill-seeking blockheads and softhearted wizards with nagging headaches always ignore.
"You sure?" I asked Mouse. "It went in there?"
Mouse circled the fence, snuffling at the dry ground protected from the rain by the El track overhead. Then he focused intently on the doors and growled.
The amulet bobbed weakly, less definitely than it had a few minutes before. I grimaced and said, "It went down here, but it traveled north after that."
Gard grunted. "Crap."
"Crap," I concurred.
The grendelkin had fled into Undertown.
Chicago is an old city—at least by American standards. It's been flooded, burned down several times, been constructed and reconstructed ad nauseum. Large sections of the city have been built up as high as ten and twelve feet off the original ground level, while other buildings have settled into the swampy muck around Lake Michigan. Dozens and dozens of tunnel systems wind beneath its surface. No one knows exactly how many different tunnels and chambers people have created intentionally or by happenstance. And since most people regard the supernatural as one big scam, no one has noticed all the additional work done by not-people in the meantime.
Undertown begins somewhere just out of the usual traffic in the commuter and utility tunnels, where sections of wall and roof regularly collapse, and where people with good sense just aren't willing to go. From there, it gets dark, cold, treacherous, and jealously inhabited, increasingly so the farther you go.
Things live down there. All kinds of things.
A visit to Undertown bears more resemblance to suicide than exploration, and those who do it are begging to be Darwined out of the gene pool. Smart people don't go down there.
Gard slashed a long opening in the fence with her ax, and we descended crumbling old concrete steps into the darkness.

Side Jobs, Heorot, Page 123-124