X the Unknown is a well-crafted imitation of the Quatermass British sci-fi pictures of the 1950s. A group of soldiers on maneuvers in Scotland stumble across a gravel pit which emanates an unusual amount of radiation. Several deaths occur before the radioactive material is mysteriously stolen. Researcher Dr. Adam Royston (Dean Jagger) speculates that the thief is some sort of inhuman monstrosity dwelling at the Earth's core. He points out that past radioactive disturbances have been occurring at 50-year intervals, each followed by sudden deaths and the disappearance of the material. Royston suggests that the unknown monster has been resuscitated by humankind's recent atomic experiments. Sure enough, the monster manifests itself as a huge slab of glowing radioactive mud (laugh now if you must -- you won't laugh when you see it). X the Unknown works well within its limited budget; unfortunately, many TV prints have been truncated, robbing some of the best horrific moments of their full impact.