A 750,000 gallon tank was created to shoot underwater photography.
The film claims to have some basis in truth because new micro-ecologies have been discovered in deep caves, especially in Romania. According to film consultant Dr. Christi Lascu (acclaimed speleologist and Romanian editor of National Geographic magazine), 35 brand new species were found in the Movila Caves in the late-1980s. Among the species found was a 10cm centipede with a venomous bite. There was nothing the size of the creature in the movie, but in theory there is not a limit for the size of animals living down there.
Filmed mainly in Romania, a country with over 12,000 registered caves.
Andrew Mason, one of the film's producers, says they hired some of the world's greatest cave divers as consultants in order to keep the movie as technically plausible as possible.
The V.S.S.F. tattoo stands for "Verband Schweizerischer Sprengfachleute" or "Swiss Society of Explosive Engineers". Hence why it was tattooed on the man laying explosives.