Assembled by the same production team responsible for the minor sci-fi classic Rocketship X-M, Unknown World isn't quite in the same league as its predecessor. The plot is set in motion by Dr. Jeremiah Morley (Victor Kilian), who theorizes that mankind could save itself during a nuclear attack by resettling far beneath the earth's surface. To prove his theory, Morley builds the Cyclotram, a combination drill and exploratory vehicle, with the financial assistance of playboy Wright Thompson (Bruce Kellogg), who insists upon joining the expedition to the earth's core. After several hair-raising adventures, the Cyclotram and its surviving passengers reach a cavern nearly 2000 miles beneath the surface. The cavern contains all the necessities of survival save one: the atmosphere renders anyone living within its walls sterile. Deciding that it isn't worth hiding in the center of the earth if only one generation will survive, the explorers endeavor to get back to the surface -- but who will survive this journey? The obligatory female lead in Unknown World is played by Marilyn Nash, who'd been discovered by Charlie Chaplin for the 1947 film Monsieur Verdoux.