Unknown World was made by two Hollywood special effects men, Jack Rabin and Irving Block, who are two of the film’s three producers. It was directed by Terry O. Morse, who shot the American scene in Godzilla, and written by Millard Kaufman, who also wrote Bad Day at Black Rock and The Klansman.
Victor Kilian, who plays Dr. Jeremiah Morley, lost an eye in a fight scene with John Wayne and was blacklisted for decades due to his political views, which is why he is uncredited in this. He’s invented a big tank that can drill into the center of the world, the Cyclotram, and has taken a crew of scientists through Carlsbad Caverns, Bronson Caves, Nichols Canyon and finally Pismo Beach to find a place where the human race can survive a nuclear war. Sadly, that unknown world makes everyone sterile.
Let me tell you, people were obsessed by the center of the Earth in 1951. The Shaver Mystery was maybe not in Amazing Stories any longer — editor Ray Palmer claimed a conspiracy got them forced out of the pulp magazine — yet still found an audience in Palmer’s newsletter called… The Hidden World.