Trivia

This film and another Robert Clarke/Edgar G. Ulmer production, The Amazing Transparent Man (1960), which was shot at the same time and in the same location, were originally to be distributed by a company called Pacific International. Shortly after the films were completed, Pacific International went bankrupt, and producer Clarke lost all the money he had put into it. The films were put up for auction by the film lab that processed them in order to recoup its costs. Both films were bought by American-International Pictures for a fraction of their cost, and upon release they made the company quite a bit of money. Except for his salary as an actor for two weeks' work, Clarke never saw a dime from the films.

With cooperation from the US Air Force and Texas Air National Guard, producer Robert Clarke filmed at Fort Worth's Carswell Air Force Base and the abandoned Marine Corps Air Station at Eagle Mountain Lake. The badly deteriorated buildings were used to show that the air base from which he took off was in ruins.

The inverted triangular sets of "The Citadel" were leftovers from an exhibition at the 1959 Texas State Fair. The producers arranged to use them before they were dismantled, and they expanded the triangular theme to the rest of the set design for that location. Note that even some triangular wipes were used for scene transitions.

Some of the sound effects are taken from George Pal's The War of the Worlds (1953).

Vladimir Sokoloff, who played "The Supreme", played "the old man" in The Magnificent Seven (1960), which was also released in 1960. Although he played a Mexican in that film, he was actually from Russia and his Russian accent can be heard in this film. He played a wide variety of Russian, French, and Spanish-speaking character in his film career which lasted more than three decades.

Arthur C. Pierce: Screenwriter appears briefly as one of the mutants escaping from the jail cell in the underground citadel.