An international consortium of scientists, operating as Project Inner Space in Tanganyika, Africa, is trying to tap into the Earth's geothermal energy by drilling a very deep hole down to the Earth's core. The scientists are foiled by an extremely dense layer of material. To penetrate the barrier and reach the magma below, they intend to detonate an atomic device at the bottom of the hole. The leader of the project, Dr. Stephen Sorenson, who is secretly dying of cancer, believes that the atomic device will burn its way through the barrier, but the project's chief geologist, Dr. Ted Rampion, is convinced that the lower layers of the crust have been weakened by decades of underground nuclear tests, and that the detonation could produce a massive crack that would threaten the very existence of Earth. The atomic device is used and Rampion's fears prove justified, as the crust of the Earth develops an enormous crack that progresses rapidly along a fault line, causing earthquakes and tidal waves along its path. Rampion warns a committee of world leaders that the crack is capable of extending beyond the fault, and that should it encircle the Earth, causing the land masses to split, the oceans would be sucked in, generating steam at high enough of a pressure to rip the Earth apart. Sorenson meanwhile discovers that there was a huge reservoir of hydrogen underground, which turned the small conventional atomic explosion into a huge thermonuclear one that was millions of times more powerful. Another atomic device, lowered into the magma chamber of an island volcano in the path of the crack, is used in the hope of stopping the crack, but it only reverses the crack's direction. Eventually the crack returns to its starting point at the test site, and a huge chunk of the planet outlined by the crack is expected to be thrown out into space. Sorenson remains at the undergroundcontrol center to record the event, despite pleas by his wife Maggie to evacuate with the rest of the project staff. She and Rampion barely escape in time to observe the fiery birth of a second moon. Its release stops the crack from further splitting the Earth.
Shooting took place in and around Madrid, which was chosen for its lower production costs. Production lasted about seven weeks. The film's technical adviser was producer Glasser's neighbor, a geologist.
Reception
Variety wrote that it was more believable than the usual science fiction premise and praised its special effects. Howard Thompson of The New York Times called it "the best science-fiction thriller this year". Time Out London called it "awesomely credible" and described the ending's imagery as disturbing.
