X the Unknown is a 1956 British science fiction horror film directed by Leslie Norman and starring Dean Jagger, Leo McKern and Edward Chapman. It was made by the Hammer Film Productions company and written by Jimmy Sangster, at the suggestion of Anthony Hinds. This was the first film Sangster ever scripted, and its success started his entire screenwriting career. The film is significant in that "it firmly established Hammer's transition from B-movie thrillers to out-and-out horror/science fiction" and, with The Quatermass Xperiment (1955) and Quatermass 2 (1957), completes "an important trilogy containing relevant allegorical threads revealing Cold War anxieties and a diminishing national identity resulting from Britain's decrease in status as a world power".

Plot

In a deserted field in rural Scotland, soldiers take turns familiarizing themselves with how to use a Geiger counter when an explosion occurs. One soldier dies of radiation exposure while another is badly burned. At the site, there is a Y-shaped crack in the ground with no apparent bottom. Dr. Royston of the Atomic Energy Laboratory is called in to investigate. He is joined by "Mac" McGill, who runs security at the UK Atomic Energy Commission. That night, a local boy witnesses an horrific off-camera sight. He dies the next day of radiation burns. Royston investigates and comes upon a tower occupied by an old man in possession of a canister left over from previous radiation experiments. In a local hospital, a young doctor collapses and melts after witnessing the same off-screen horror as the boy.