The movie takes place on Pellucidar, a fictional Hollow Earth invented by Edgar Rice Burroughs for a series of action adventure stories.
The last film produced by Amicus, Hammer's chief rival during the 1960's and 70's. Filmed January 20-March 1976, on the heels of Hammer's final horror feature "To the Devil a Daughter (1976)."
This popular film became the 18th most profitable British film of 1976.
The film opened on Edgar Rice Burroughs's birthday, 1 Sep, in theaters and drive-ins throughout Southern California.