PARIS - Walking through the Paris catacombs, it's hard to believe Hollywood hasn't mined this wonder more on the big screen.  There have been films that have tried, 2007's "Catacombs" being one of them, but its a rare few that have ever gotten access to shoot in the actual underground tunnels and quarries.  The upcoming thriller "As Above/So Below" is the exception.

Directed by John Erick Dowdle ("Devil," "The Poughkeepsie Tapes") from a screenplay by Dowdle and his brother Drew, "As Above/So Below" introduces audiences to Scarlet Marlowe (Perdita Weeks), a gutsy archeologist on a global search for an ancient artifact.  When she determines this potentially magic object may lie beneath the City of Lights in the Catacombs of Paris she recruits an old flame (Ben Feldman, soon to be seen on NBC's "A to Z") and a few local tunnel experts to check it out.  When they are forced to take a notoriously haunted (aka "evil") tunnel there world literally gets turned upside-down.

Speaking to the Dowdle brothers in the actual catacombs (one of the more unique locales this writer has ever conducted a video interview in), the duo admitted to a strange sense of Deja vu having spent almost two months filming in the same tunnels just a year ago.  While the credit for suggesting a movie in the catacombs goes to Legendary Pictures CEO Thomas Tull, the Dowdles admitted they didn't know what they were getting into until they first scouted the expansive catacomb system.

"We would be crawling on our bellies for 100 yards at a time. It was genuinely scary.  It was like a nightmare," John Erick says. "And one of the people we were with totally lost his mind and we were like, 'O.K., we've got to capture this feeling in the movie.'"

The Dowdles and their location scouts reviewed every underground space in and around Paris.  While they had to wait for permissions from the government, "As Above" eventually became first production to ever get permission to shoot in the off limits areas of the catacombs. Drew Dowdle notes, "This system right here was the first big victory we got. There were some that were very important for us to have in the movie that were extremely difficult to get permits for."

John Erick adds, "The south system it's called. That was a really tricky one we had to have."

One condition of the permits were that the production and crew would be relatively small. The Dowdles therefore used a "skeletal crew" and put the actors to work by having them wear cameras and lights on their heads.  And while they places they shot in were publicly known (there are still hundreds of unmapped tunnels), they didn't prohibit them from wade through water and crawling through small enclosures to get where they needed to be.  This is when acting is fun, right?

When casting a movie like this it goes without saying that every potential actor got a question or two about their own feelings about working in enclosed spaces.  For the most part the cast was honest. Again, for the most part.

John Erick reveals, "Both with actors and crew we had to say 'You aren't claustrophobic, right?' And even then, 'Just to make sure, let's do a wardrobe test and do it underground.'  We realized one of our cast members definitely wasn't very comfortable down here. Let's write that in. Let's make him claustrophobic. He was a real trooper. He was down here and he'd have to take a moment and calm himself. You'd feel his anxiety."

"That particular actor had the most claustrophobic scene in the movie and he seemed to really enjoy what it did for his performance," Drew adds. "His performance was so solid and I don't think it was much acting."

Now, who that actor actually was? Well, that might be the biggest secret of the movie.

For more from the Dowdle brothers on "As Above/So Below" including how they got the right to set a car on fire in a catacombs quarry watch the video embedded at the top of this post.

"As Above/So Below" opens nationwide on August 29.