Diana Urban’s Under the Surface hits shelves August 13, an epic survival-thriller about four teens who get lost in the Paris catacombs for days. If that sounds TERRIFYING to you, you’re 100% right, and it doesn’t end there. We asked Diana Urban to share the most chilling facts she uncovered about the Paris catacombs while writing. Scroll down to see what she said (if you dare!) along with some sneak peek excerpts.

And don’t forget to preorder your copy!

Hi, I’m Diana Urban, author of the survival thriller Under the Surface about a girl named Ruby who gets lost in the Paris catacombs with three friends for days while aboveground the boy she loves, Sean, is desperate to find her. 

This book was inspired by true events, so Penguin Teen challenged me to share some of the most fascinating facts I learned about the catacombs while researching for this book, and how I dropped those chilling nuggets into the book itself. But be warned: there are some spoilers below!

Fact #1: When in Paris, there’s a city’s worth of bones under your feet

When people think of the Paris catacombs, they might imagine the small tourist section where you descend 131 steps to follow winding yet clearly-marked corridors leading to “The Empire of Death,” where you can view corridors lined with human bones arranged in artistic patterns.

But this ossuary is only a small part of the vast network of 300km of underground tunnels that span much of the city, home to over six million dead Parisians. Most of it’s illegal to traverse because it’s so dangerous — there are cave-ins, flooded corridors, and the constant threat of getting lost if you’re not with an experienced guide.

In Under the Surface, Ruby gets lost in those off-limits sections, and she explains it best when she first meets Julien, an alluring Parisian boy who takes her and her friends deep underground:

“We’ve been dying to see the catacombs—the intricate web of tunnels beneath the city where the skeletal remains of six million long-dead Parisians line miles and miles of passageways in artistic arrangements. It has to be one of the creepiest things you can see on the entire planet, which obviously means I have to see it. Mr. LeBrecque added it to our itinerary after I begged and pleaded, but we’d be visiting the small touristy section—the only bit open to the public. Most of the other entrances scattered throughout Paris have been sealed for ages, and the only accessible ones are secret from everyone except for—

“You’re a cataphile, aren’t you?” I ask Julien, breathless. Suddenly his outfit makes sense. 

Selena sputters a laugh. “Did you just call him a pedophile?” 

“A cataphile,” Olivia pipes up. “It’s what they call the explorers who meet up in the catacombs.” Of course she’d know about them, too. “Illegally, I might add. Isn’t it dangerous down there?” 

“How’s it dangerous?” Selena asks. 

“It’s not if you know what you’re doing,” says Julien, “and where you’re going. And I do.” 

Fact #2: Thieves used to pose as tour guides

When I first toured the catacombs in 2015, our guide shared how in the 1800s, thieves would pose as tour guides to lure people deep into the catacombs, then rob them and leave them to find their way out alone. That’s why there’s now a thick black line painted along the ceiling of the official tour route, so if anyone ever found themselves lost without a map or guide, they could follow the line to the exit. They called it the “lifeline” because it was your chance to come out alive.

Sean sees this lifeline while touring the catacombs with his class, as their teacher tries to keep everyone occupied while the French police search for the missing girls.

“Did anyone notice this black line?” Guillaume asks. 

I’d vaguely noticed it winding through our route along the scabrous ceiling, reminding me of the red brick Freedom Trail back in Boston, so I absentmindedly raise my hand. 

Always a mistake. 

He calls on me. “Do you know what it’s for?” 

“Uh . . .” I stiffen, hating getting put on the spot. “Is it . . . so people don’t get lost?” 

“Very good,” he says. I relax my posture. “It’s called the lifeline. When the catacombs first opened to the public in the eighteen hundreds, none of these lights were installed yet, and you could easily take a wrong turn into an enormous, pitch- black maze. Back then, grifters would bring people down here and threaten to leave them to find their way out alone . . . unless they paid a price.” He rubs his fingers together, eyes glinting. 

It’s partly this anecdote that originally inspired Under the Surface. I thought how scary that would be to get lost in that dark maze with only one flashlight, the battery like a ticking timer. Wouldn’t that make for a terrifying story? And what would happen if a group of teens got lost down there? What if one of the girls was in love with a boy on the surface who had no idea where she went? So the idea was born.

(And fortunately, my own tour guide was not a thief.)

Fact #4: Teenagers got lost in the catacombs in 2017

Over the next couple of years as the characters formed in my mind, I researched more, learned more about cataphiles (more on them below), and watched YouTubers share firsthand accounts of traversing the dark corridors — apparently, some thieves still pose as tour guides today.

Then in June 2017, I saw a news story about teenagers who were rescued from the Paris catacombs after being lost for three days. Here’s CNN’s coverage.

This story didn’t gain much traction, and if you try to research it, you won’t find much. I never attempted to reach out to the parties involved; their experience was, I’m sure, quite harrowing, and I hope they’ve made a full recovery in peace. But seeing this brief snippet was validating: the premise was realistic and not too far-fetched, as it just happened.

Fact #5: Cataphiles explore the off-limits sections

Cataphiles like Julien are urban explorers who venture through the off-limits sections of the catacombs to host parties and raves, graffiti the stone walls, swim in the underground lakes, and find hidden alcoves no one’s set foot in in hundreds of years. As Julien says, “Coming down here . . . it’s like a drug. An addiction. The rush of exploring the unknown, of finding rooms nobody has entered in centuries—there’s nothing else like it.”

You see these elements incorporated into the book—the party Julien meant to take the girls to before they get lost, the graffiti in the off-limits sections, the underground lake where the lost group must hold their breath for over a minute to swim across.

The cataphiles abide by a code, including doctrines like “what comes down must go up.”

“Watch it.” Julien points at the plastic seal she accidentally dropped. “What comes down must go up.” 

“Sorry, sorry.” She [Olivia] picks it up, harried, not used to getting things wrong. 

“These are sacred halls. Let’s not disrespect the dead by leaving our trash down here with them.” 

The dead. I peer down the corridor. They’re down here somewhere. 

They also play music as they explore so other cataphiles can hear they’re coming and know they’re friendly. And music—or the lack thereof—is also a clue that the sounds the girls and Julien hear from someone who’s chasing them is not actually coming from cataphiles, but someone much more sinister…

Fact #6: Cataphiles are actually… not so creepy

The police play cat-and-mouse with illegal explorers, sealing the entrances throughout Paris to limit access to old mines—manhole covers, old doorways in the Metro system, trapdoors in old basements, and so forth. But according to my cataphile guide, the police mostly want to keep casual tourists away. They don’t mind the cataphiles’ presence in the catacombs since they know the layout best and help monitor the structural integrity of the corridors. My guide literally noticed, inspected, and photographed a crack in the middle of our tour so she could report the damage… with lots of unnerving muttering about how “that wasn’t like that last week.”

The cataphiles also help keep tourists from getting lost. All those rumors you hear about cataphiles grabbing your maps and running away? Maybe that happens. But if they’re a real cataphile, they’ll come back to lead you to the surface. They’ll only scare you to make sure you don’t try risking your own neck to come back again.

The creepy rumors certainly were great inspiration for the sinister presence that chases Ruby and company. But in the book, Sean notes how the cataphiles help the police investigation… and I think they would in real life, too. (To say more than this would venture too far into spoiler territory!)

Fact #7: How they got all those bones down there

The tunnels that comprise the catacombs weren’t always underground ossuaries, but originally limestone quarries dating back to the late 13th century. Miners would extract the stone out of enormous well shafts, and this stone is thought to have been used in some of Paris’s most famous buildings, including Notre Dame and the Louvre palace.

The mines were largely forgotten about until the late 1700s when streets of Paris started collapsing into the old tunnels. Literally! On December 17, 1774, an entire city street fell into a 20-meter deep hole known as the “Mouth of Hell.” This unfortunate incident triggered an inspection and fortification project of the old mines.

Around the same time, Paris was experiencing a rising death toll the cemeteries couldn’t keep up with, causing them to bury bodies in mass graves. During a particularly bad storm, the graves started flooding into neighboring shops’ basements—again, literally, corpses and all. It created such a critical public health hazard, city officials decided to relocate the bones, and since it was too expensive to shuttle them all outside the city, they decided to drop them into the old mines.

Once again, literally.

They dropped them down the same wells the miners used to use to extract limestone. 

Underground, the workers fortifying the tunnels collected the bones and organized them into ossuaries, using their usual construction methods: small bones piled in the back, bigger pieces like femurs and skulls up front. They took pride in their work, creating intricate patterns—stacks and zigzags, crosses and hearts—that Ruby and company often run across while outrunning the sinister presence chasing them.

Since I had to choose what information would be most relevant to the plot, not all of this made it into the book, but Olivia does explain some of it when the group comes across a creepy mural showing Paris splitting in two, its iconic buildings collapsing into an enormous tear in the earth.

“Paris did actually split open like this once.” 

“Yeah, right,” says Val. 

“No, it’s true,” says Olivia. “A mile-long trench opened up one afternoon—December 1774, I think—and all the houses on that stretch collapsed into it. They called it the Mouth of Hell.”

“But if there are no fault lines . . .” I trail off. 

“It wasn’t an earthquake. It was actually because of this place. The catacombs.” She tilts her head. “Only, they weren’t catacombs yet. They were originally mines. They took a ton of limestone from down here to build the city, even buildings like Notre Dame, but they didn’t properly fortify these tunnels afterward. So buildings started collapsing into them.” 

“It wasn’t until they started moving the dead into the tunnels that the city stabilized,” says Julien. 

“Yeah, that’s around when they started bringing down all these bones.” Olivia motions to the column. “The fortification work reminded them Oh, hey, there’s all this empty space right under our feet; perfect place to dump the bodies from the overflowing cemeteries.” 

Fact #8: Julien’s favorite crepe stand actually exists

Okay, this one will only make sense if you’ve read the book… and for those who have, this will be more heart-wrenching than chilling. 

I run my tongue over my teeth, brainstorming how to cheer him up. I’m no good at small talk—not with strangers, anyway. “So, what’s your favorite spot in Paris?” 

He grins. “Oh, easy. There’s this little crepe stand right beside Notre Dame. I get one with strawberries and Nutella, so warm and gooey, the edges so crisp, ugh, it is perfection.” He clasps his stomach. My mouth would water any other time, but after breathing in stale air that reeks of damp dust for hours, it feels dry and gritty. “I like to sit on a bench right there with that magnificent view, early in the morning, when the sky is clear and it is not so busy. There is no better place, not in Paris, not in the world.” 

“I love that,” Val says, but I scoff. 

Julien peers at me again. “What?” 

My cheeks warm. His answer reminds me so much of what Tyler told me about Paris that night in his bedroom. “It’s just—That sounds so cliché. So touristy.” 

He gives me a little shove. “Look at you, judging me.” The corners of his eyes crinkle. “Locals can enjoy what the tourists do. That’s how it becomes a tourist attraction to begin with, yes? The most appealing spots become the most popular.” Julien brandishes his arms. “And up until yesterday, this was my favorite spot.” 

“Sorry we ruined the death maze for you,” Selena mutters, heaving a sigh. “We’re never going to find a way out, are we?” 

So the next time you head to Paris, look for one of these crepe stands around the perimeter of Notre Dame’s courtyard. And get one for yourself. For Ruby. And for Julien.

But all facts aren’t present in fiction…

As much as I tried to incorporate historical and present-day accuracies into the book, I also took many artistic liberties, including the presence of the [redacted] who chase Ruby and company through the catacombs, as well as the layout of the corridors they traverse. Because this route doesn’t exist, I commissioned a map artist, Danny Ride, to bring this vision of the catacombs to life. Here’s a sneak preview; you’ll be able to find this map in the book when it comes out on August 13 — the intricate details are brilliant, and I can’t wait for you to see them up close!

Preorder Under the Surace