They turn out to be a pair of siblings. They have the same brown hair, pointed chins, and gray-blue eyes. But otherwise, they look like shadows at different times of day—Anton is as tall and thin as a skeleton, while Annette is short and square.

Pauline kisses each of them twice, once on each cheek, then turns to my parents.

“If you are ready, we should leave. We will start with the Catacombs.”

“Yes,” says Mom, brushing sugar from her lap. “The ghosts of Paris await.”

“What’s a catacomb?” I ask as we step outside.

“It’s a kind of graveyard,” says Dad.

“Like Greyfriars?” I ask, thinking of the hilly cemetery nestled in the heart of Edinburgh.

“Not exactly,” he says. “It’s—”

“Don’t ruin the surprise,” says Mom, which makes me intensely nervous. Mom’s idea of a surprise has always been less Happy birthday and more Look at this vaguely nightmarish thing I found in the backyard. “Just wait, Cass,” she says. “The Paris Catacombs are one of the most famous places in the world.”

“At least she didn’t say most haunted,” muses Jacob, right before Mom adds, “And definitely one of the most haunted.”

Jacob sighs. “Of course.”

We take the Metro across the city and get off at a stop called Denfert-Rochereau.

Outside, I notice a placard on a building’s stone wall that says 14e: the number of the neighborhood we’re in. As we walk, I keep my eyes peeled for a graveyard, but all I see are normal buildings. And yet I know we’re getting closer because I can feel the tap-tap-tap of ghosts getting louder with every step.

The Veil ripples around me, and the beat shifts from my chest to my feet, a heavy bass drumming through the street. Haunted places don’t just call to me. They drag me in like a fish on a line. There’s no hook, only a thread, wisp-thin but strong as wire, connecting me to the other side.

My parents, Pauline, and the crew come to an abrupt stop in front of a small green hut. It’s plain and inconspicuous, more like a newspaper stand than a place for the dead. In fact, it doesn’t look large enough to hold more than one or two coffins. At first I think we must be in the wrong place, but then I see the copper plaque nailed to the painted wood.

ENTRÉE DES CATACOMBES.

“Huh,” I say. “I thought the Catacombs would be … bigger.”

“Oh, they are,” says Dad, pulling out one of his guidebooks. He shows me a map of Paris, and then turns the page in front of it. A filmy sheet of paper settles over the map, its translucent surface traced with red lines.

Slowly, I realize what I’m looking at. I also realize why I felt so weird as we walked.

The Catacombs aren’t in this little green hut.

They’re under our feet. And judging by the map in Dad’s hand, they’re under a lot of people’s feet. The Catacombs are a coil of tunnels twisting back and forth on themselves beneath the city.

We approach the door, but a sign on the wall announces that the Catacombs are closed.

“Oh, too bad,” says Jacob. “We’ll just have to come back another time …” He trails off as a man in a security uniform appears, unlocking the entrance to the little green shack and ushering us through.

Inside, there’s a pair of turnstiles, like the beginning of a roller-coaster ride.

We pass through and find ourselves at the top of a spiral staircase wide enough for only one person at a time. It plunges down out of sight. The tunnels below seem to exhale, sending up a draft of cool, stale air, along with a wave of anger, and fear, and restless loss.

“Nope,” says Jacob, shaking his head.

This is a bad place, and we can both feel it.

I hesitate as the Veil tightens its grip, calling me down even as something deep in my bones tells me to stay put, or even better, to run.

Mom looks back over her shoulder. “Cass? You okay?”

“Just tell them you’re too scared,” says Jacob.

But I’m not, I think. I am scared, but there’s a difference between being scared to do something and being too scared to do it. Plus, I think, clutching my camera, I have a job to do. And I don’t even mean ghost-hunting. My parents asked for my help. I don’t want to let them down.

And so I propel myself forward and take the first step.

“Everything about this is terrible,” says Jacob as we descend, down, down, down into the tunnels under Paris.

I used to have this one bad dream.

I was trapped in a room, deep under the earth. The room was glass, so I could see the dirt on every side, pressing against the walls.

The dream was always the same. First I would get bored, and then I would get nervous, and then, at last, I would get scared. Sometimes I would bang on the walls, and sometimes I would sit perfectly still, but every time, no matter what I did, a crack would form in the glass.

The crack would spread and spread, up the walls and overhead, until bits of dirt came through and then, just as the ceiling shattered, I’d wake up.

I haven’t thought of that dream in years.

But I think about it now.

The spiral stairs are a tight coil, so we can’t see more than a full turn at a time, and they just keep going, and going, and going.

“How far down are the Catacombs?” I ask, fighting to keep the fear out of my voice.

“About five stories,” says Dad, and I try not to think about the fact that the Hotel Valeur is only four stories tall.

“Why would you put a graveyard underground?” I ask.

“The Catacombs weren’t always used as a graveyard,” explains Dad. “Before they became an ossuary, the tunnels were simply stone quarries that ran beneath the growing city.”

“What’s an ossuary?” I ask.

“It’s a place where the bones of the dead are stored.”

Jacob and I exchange a look. “What happened to the rest of them?”

Mom chuckles. It doesn’t make me feel any better.

“The bodies in the Catacombs were transferred here from other graves,” explains Pauline.

Transferred.

Meaning dug up.

“Oh, I do not like this,” says Jacob. “I do not like this at all.”

“Today,” says Dad, “the Catacombs are home to more than six million bodies.”

I nearly trip on the steps. I must have heard him wrong.

“That’s three times the living population of Paris,” adds Mom cheerfully.

I feel a little queasy. Jacob glowers at me as if to say, This is your fault.

We finally reach the bottom of the stairs, and the Veil washes up around me like a tide, dragging at my limbs. I push back, trying to keep my footing as Jacob draws closer.

“We are not crossing here,” he says, all the humor gone from his voice. “Do you hear me, Cass? We are not. Crossing. Here.”

He doesn’t have to tell me.

I have no desire to find out what’s on the other side of this particular Veil.

Especially when I see what’s ahead of us.

I’d been hoping for a large space, like one of those giant caves with stalactites—stalagmites? I can never remember which is which—but instead there is only a tunnel.