"What do you mean, 'Undertown'? I've never heard of it before."

"I was probably working here for five or six years before I did," I said. "You have to understand the history of Chicago. How they did things here."

Billy folded his arms. "I'm listening."

"The city is a swamp," I said, still searching for a means of opening the door with my fingertips. "We're darn near level with Lake Michigan. When they first built the place, the town kept sinking into the muck. I mean, every year it sank lower. They used to build streets, then build a latticework of wood over them, and then another street on top of that, planning on them slowly sinking. They planned houses the same way. Built the front door on the second floor and called it a 'Chicago entry,' so when the house sank, the front door would be at ground level."

"What about when the street sank?"

"Built another one on top of it. So you wound up with a whole city existing under the street level. They used to have a huge problem with rats and criminals holing up under the streets."

"But not anymore?" Billy asked.

"The rats and thugs mostly got crowded out by other things. Became a whole miniature civilization down here. And it was out of sight of the sun, which made it friendly space for all the night-crawling critters around."

"Hence, Undertown," Billy said.

I nodded. "Undertown. There are a lot of tunnels around Chicago. The Manhattan Project was housed in them for a while during World War Two. Did all that atomic bomb research."

"That's cheerful. You come down here a lot?"

I shook my head. "Hell, no. All kinds of nastiness lives down here."

Billy frowned at me. "Like what?"

"Lots of things. Stuff you don't often see on the surface. Things even wizards know almost nothing about. Goblins, spirits of the earth, wyrms, things that have no name. Plus the usual riffraff. Vampires sometimes find lairs down here during the day. Trolls can hide here too. Molds and fungi you don't get in most of the natural world. You name it."

Billy pursed his lips thoughtfully. "So you're taking us into a maze of lightless, rotting, precarious tunnels full of evil faeries and monsters."

I nodded. "Maybe leftover radiation, too."

"God, you're a fun guy, Harry."

"You're the one who wanted in on the action." My fingers found a tiny groove in the wall, and when I pressed against it a small, flat section of stone clicked and retracted. The switch had to have triggered some kind of release, because the section of wall pivoted in the center, turning outward, and forming a door that led into still more dank darkness. "Hah," I said with some satisfaction. "There we go."

Billy pressed forward and tried to step through the door, but I put a hand on his shoulder and stopped him. "Hang on. There are some things you need to know."

Billy frowned, but he stopped, listening.