X: The Unknown

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?