Unknown World

Dr. Jeremiah Morley (Victor Kilian) is concerned about an imminent nuclear war in this 1951 film, so he organizes an expedition of scientists who use an atomic-powered machine, capable of drilling through earth and stone, known as the Cyclotram, to find an underground environment where people could escape from nuclear obliteration. In other words, he's worried about Dr. Strangelove's so-called "mineshaft gap."
The first 13 minutes of the film are almost straight exposition, which is not a good thing in a 74 minute film. It's not a terribly suspenseful film. Most of the viewer's time is spent waiting for something interesting to happen which never does. The special effects aren't great, but they aren't laughingly bad either, so I guess that's a plus. It's too bad this wasn't film wasn't shot in colour as parts of it were filmed at Carlsbad Caverns and Bronson Caves.
In the end, the scientists find an enormous underground expanse with lots of air, its own ocean and phosphorous light. Huzzah.
Victor Kilian is clearly one of the leads actors in the film as Morely, yet he received no screen credit as he was "blacklisted" during the MacCarthy "Red-scare" after filming was completed. But he continued to have a long career in television until he was beaten to death by robbers in 1979. Director Terry O. Morse had a long career as an editor lasting almost 50 years, but he also directed a number of films. His next directoral effort after this would the American version of the first Godzilla movie.