For their found-footage horror film As Above, So Below, which premieres Friday from Legendary Pictures, brothers John Erick and Drew Dowdle received unprecedented access to the Catacombs of Paris, which hold the remains of about 6 million people.

In fact, as the filmmakers revealed during their visit to the CBR Yacht at Comic-Con International, they were the first production to shoot in not only in the public areas of the underground cemetery, but in the restricted sections as well.

"Some of our locations, we literally got permission the night before we were shooting," Drew Dowdle, who co-wrote and produced the film, told CBR TV. "It was always, like, tentative permission that we would get made official the night before. It was very touchy. It wasn't a monetary thing. It was very ... you know, they're historical sites, and we were a genre film coming in -- an American genre film coming in. [...] The French don't really do -- they're such a cinephile country, but they don't really do horror films, so this took a little convincing."

As Above, So Below follows a team of explorers that ventures into the twisting tunnels, only to discover the dark secret that lies beneath Paris' city of the dead.

Filming in the actual catacombs contributed extensively to the look of the film, but it also placed dramatic restrictions on the cast and crew.

"The way we lit the films, the actors actually lit the whole film with their own headlamps," said director and co-writer John Erick Dowdle. "We had a light on the camera, and that actually was the only light we used in some of the pivotal scenes of the movie. The blocking, you know -- walkie-talkies don't work through those walls, cell phones don't work down there, monitors ... you can't run enough cable. The actors would be running down the corridors of the catacombs, and we'd be there with the wireless monitor 20 feet behind them with no headlamp on, in the dark -- you just keep an arm up so the ceiling lowered at any point you'd take it in the arm and not in the face. It really was guerrilla filmmaking at its finest."

It was also more than a little dangerous, they admit. "We all took some head divots, for sure, my God." Drew Dowdle said. "I took one that didn't heal for a good six months."