When the tips of my fingers touched the door, it swung open as if my skin was somehow the genetic recognition, the key that opened the door. Link smashed into me, and Liv tumbled on top of both of us. I cracked my head against what seemed like stone as I hit the ground. I felt so dizzy, I couldn't see anything. When I opened my eyes, I was staring up at a streetlamp.

"What happened?" Link sounded as disoriented as I was.

I felt around the edge of the stones with my fingertips. Cobblestones. "I just touched the door, and it opened."

"Amazing." Liv stood up, taking it al in.

I was lying in a city street that looked like London or an old town right out of a history book. Behind me, I could see the round doorway, at the road's end. There was a brass street sign next to it that said WESTERN DOORWELL, CENTRAL

LIBRARY.

Link sat up next to me, rubbing his head. "Holy crap. This is like one of those al eys where people got hacked up by Jack the Ripper." He was right. We could have been standing in the mouth of an al ey in nineteenth-century London. The street was dark, lit by only the dim glow of a few lampposts. The al ey was framed on both sides by the backs of tal brick row houses.

Liv stood up and made her way down the deserted cobblestone street, looking up at an old iron street sign: THE KEEP. "That must be the name of this particular tunnel. Unbelievable. Professor Ashcroft told me, but I never imagined. I suppose books couldn't real y do it justice, could they?"

"Yeah, it looks nothin' like the postcards." Link pul ed himself to his feet. "Al I wanna know is, where'd the ceilin' go?" The curved arch of the tunnel's ceiling was gone, and in its place was a dark evening sky, as big and real and ful of stars as any sky I'd ever seen.

Liv pul ed out her notebook and started writing. "Don't you get it? These are Caster Tunnels. They're not some supernatural subway system, so Casters can creep around under Gatlin borrowing library books."

"Then what are they?" I ran my hand along the rough brick on the side of the nearest building.

"More like roads to another world. Or, in a way, a whole world al to themselves."

I heard something, and my heart jumped. I thought Lena was Kelting, reconnecting with me. But I was wrong.

It was music.

"Do you hear that?" Link asked. I was relieved. For once, the music wasn't coming from inside my head. It was coming from the end of the al ey. It sounded like the Caster music from the party at Ravenwood last Hal oween, the night I saved Lena from Sarafine's psychic attack.

I listened for Lena, felt for her, remembering that night. Nothing.

Liv checked her selenometer and wrote something else in her notebook. " Carmen. I was transcribing one yesterday."

"English, please." Link was stil staring up at the sky, trying to figure it al out.

"Sorry. It means 'Charmed Song.' It's Caster music."

I took off, fol owing the sound down the al ey. "Whatever it is, it's coming from down here."

Marian had been right. It was one thing to wander through the damp tunnels of the Lunae Libri, but this was something entirely different. We had no idea what we had gotten ourselves into. I already knew that much.

As I walked down the al ey, the music grew louder, the cobblestones smoothed their way into asphalt beneath my feet, and the street changed from Old World London to modern-day slum. It was a street you could find in any big city, in some forgotten run-down neighborhood. The buildings looked like abandoned warehouses, iron grating covered the shattered windows, and the remnants of broken signs blinked fluorescent light into the darkness. There were cigarette butts and trash al over the street, and a strange sort of Caster graffiti -- symbols I couldn't begin to understand -- on the sides of the buildings. I pointed it out to Liv. "Do you know what any of that means?"

She shook her head. "No, I've never seen anything like it. But it means something. Every symbol in the Caster world has significance."

"This place is even freakier than the Lunae Libri." Link was trying to play it cool in front of Liv, but he was having a hard time pul ing it off.

"Do you wanna go back?" I wanted to give him an out, but I knew he had as much of a reason to be down here as I did. His reason was just blonder.

"Are you cal in' me a wuss?"

"Shh, shut up --" I heard it.

The Caster music drifted through the air, the seductive melody replaced by something else. This time, I was the only one who could hear the words.

Seventeen moons, seventeen fears,

Pain of death and shame of tears,

Find the marker, walk the mile,

Seventeen knows just exile ...

"I hear it. We must be close." I fol owed the song as it looped over and over in my head.

Link looked at me like I was crazy. "Hear what?"

"Nothing. Just fol ow me."

The huge metal doors lining the filthy street were al the same, dented and scratched, as if they'd been attacked by an enormous animal or something worse. Except for the last door, the one with Seventeen Moons playing inside. It was painted black and covered with more Caster graffiti. But one of the symbols looked different, and it wasn't spray-painted on the door. It was carved into it. I ran my fingers over the cuts in the wood. "This one looks different, almost Celtic."

Liv's voice was a whisper. "Not Celtic. Niadic. It's an ancient Caster language. A lot of the older scrol s in the Lunae Libri are written in it."