American Sergeant Joe Cramer, also finds the cavern and alerts his small reconnaissance party, led by a Captain Wilson and accompanied by a British war journalist and officer, who still likes to be addressed as "General" Braithwaite.

The group manages to hold together for many months while seeking a way out of the cavern, but the confinement, dwindling supplies, and personality conflicts take an increasing toll on their mental states and united purpose.

Cramer discovers Wilson and Braithwaite had been boozing it up and confiscates the brandy, putting it under lock and key, issuing it only occasionally on special occasions like Christmas dinner.

Overcome by his alcoholism, creeping insanity, and growing claustrophobia, Braithwaite commits suicide by pulling the pin and falling on a hand grenade in one of the chambers that had been used to store explosives.

Although the screenplay is credited to Jack Davies and Michael Pertwee, some confusion has existed about whether those writers were serving as fronts for the once-blacklisted screenwriter Dalton Trumbo.

Working with a small budget by Hollywood standards, but still larger than most films he had directed, Ulmer began shooting in the Postojna Caves in Slovenia (Yugoslavia at the time) but was forced to relocate to Trieste, Italy, where an interior cavern set was constructed.