X: The Unknown (1956)
British Army radiation drills at a remote Scottish base attract a subterranean, radioactive entity of unknown nature that vanishes, leaving two severely radiation-burned soldiers... and a "bottomless" crack in the earth. Others who meet the thing in the night suffer likewise, and with increasing severity; it seems to be able to "absorb" radiation from any source, growing bigger and bigger. What is it?? How do you destroy a thing that "feeds" on energy?