Sometime around 1997, there emerged a massive Internet hoax which claimed that a Siberian borehole had penetrated the Earth's crust into Hell itself, with "proof" being an audio recording of the screaming souls of the damned. This urban legend (variously known as the "Siberian Sounds of Hell" or "The Well to Hell" hoax) featured in many tabloids, and was even cited by some Christian groups as hard proof of a real Hell. The sound effects supposedly recorded within the borehole were actually a combination of story elements from a radio broadcast "Quiet Please - The Thing on the Fourble Board", and audio lifted from this film.
Originally director Mario Bava didn't want to shoot the film outside of Italy, but after producer Alfredo Leone found the castle location in Vienna, Austria, Bava decided it was a perfect setting for the film.
According to the DVD liner notes, this was the first film ever shot aboard a 747.
At first Vincent Price was approached to star in the production but declined. Then Ray Milland was considered, but he was unable to travel to the shoot. Finally veteran actor Joseph Cotten was suggested and, to director Mario Bava's surprise, accepted..
Shot in six weeks, finishing three days ahead of schedule and under budget.
Alfredo Leone: can be seen seated behind Antonio Cantafora in the opening scenes aboard the Pan Am 747. His daughter Kathleen Leone is sitting next to him.