X the Unknown is a 1956 British science fiction horror film directed by Leslie Norman and starring Dean Jagger and Edward Chapman.

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".

The same evening, when two soldiers mysteriously die while guarding the Y-shaped crack, Royston's colleague Peter Elliott volunteers to be lowered into the pit to investigate.

But every 50 years there is a tidal surge that these creatures feel, causing them to reach the surface and find "food" in the form of radioactive sources.

Although Losey did begin shooting the film and some of his footage is included in the final cut, he was replaced by Leslie Norman due to illness.

[11] Nonetheless, the American distribution deal between Hammer and RKO fell through due to the latter company's pending demise, and the film was distributed in the U.S. by Warner Bros.[1] Monthly Film Bulletin wrote: "X the Unknown intriguingly suggests a new addition to the science fiction repertoire of Things, but after a series of prolonged climaxes, with its potential victims staring directly into the camera and shaking with fright, the "Unknown" finally emerges as a type of rolling rubber mattress, disappointingly unhorrific in content and appearance.

Scientific explanations for the object's arrival are disconcertingly vague; one is left with the impression that the "Unknown" has been created with the sole purpose of manoeuvring its elaborate destruction, and the script sometimes suggests this in a number of unintentionally comic lines.

The first reviewer, "Myro", wrote in 1956: X the Unknown is a highly imaginative and fanciful meller, with tense dramatic overtones which will help it along at the boxoffice. ...

Laboratory experiments in an atomic research station have an impressive, but familiar appeal, though ultimately they play a key role in the plot.

The scenes on the desolate moor, the sight of the grim atomic mass moving relentlessly towards its main target, the closeups of the radio-active victims, and the ultimate efficacy of the neutralizer combine in achieving a tense, almost horrific atmosphere.

The acting, though mainly stereotype in style, is in the same vein, with Jagger, Edward Chapman and Leo McKern leading a vigorous cast.