Así en la Tierra como en el infierno

This was the first ever production that secured permission from the French government to film in the Paris catacombs, both in the public and in the off-limits area. They got permission the night before shooting began. The air and water quality in the catacombs were tested before shooting to make sure the cast and crew would not walk through battery acid. The only entrance to one of the catacombs' main locations was through the parking lot of an hospital. There was very little use of props. The film utilizes a set of narrow, winding tunnels of the Paris catacombs, complete with real mint skeletons creepily arranged centuries ago. There was no electricity, no bathrooms or cell phone service in the tunnels. Sometimes the cast and crew had water up to their waists, and some days they had to crawl around on all fours for an hour. Several scenes were lit up by the actors themselves with their head lamps. Director John Erick Dowdle said that the actors were shooting the film 90% of the time.

The poem on Flamel's stone shouldn't rhyme in English.

On the last seconds of the end credits, you can hear a character saying "The only way out is down", followed by some of the noises heard on the catacombs earlier in the film.

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