Back in the days when "Prince Valiant" still covered most of a page in the Sunday funnies, I first saw this wonderfully creepy SF sleeper. From the opening scenes of a stretch of the Scottish moors opening up under a British soldier's feet, I was hooked. I was very lucky that my parents didn't send me to bed then(although they did right in the middle of "From Hell It Came", but that's another story); I was resolved to catch it again just to get that creepy feeling! The ominous P.O.V. shots of the "X" monster shambling through the barren woods at night, the outrageous "melting" death by hard radiation of the medical tech in the X-ray room, Dean Jagger as Dr. Royston, describing his new invention that can (theoretically) speed up decay of radioactive materials without causing flare-ups(!) simply by leeching away its' energy, and the film's creepy, near-ambiguous ending ("We did destroy it...didn't we?") showed just what you could--and can!--still do with a small budget and respect for the material.
