The film starred Dean Jagger, Edward Chapman and Leo McKern. It was written by Jimmy Sangster, and was originally to have been directed by Joseph Losey (working under the name Joseph Walton). Losey was an American director who had moved to the UK after being placed on the Hollywood blacklist. Although Losey did begin shooting the film and some of his footage is included in the final cut, when star Dean Jagger arrived on set he refused to work with a suspected Communist sympathiser. Losey was replaced by Leslie Norman, officially for health reasons.
The plot of the film sees a remote British Army base in Scotland terrorised by an unknown radioactive entity that lives underground. As the creature continues to grow in size by feeding off radioactive energy, a team of scientists are called in to investigate and stop it before it becomes too late.
A creature unknown to man, a creature the appears and disappears in the bottomless crack in the Earth, a creature that absorbs radioactivity, burning every living being close by.
"X The Unknown" is an atmospheric doomsday science fiction movie that relocates the events into the gloomy and swampy countryside of England rather than the desert as its many American counterparts used to do.
Without noise or distortion, "X The Unknown" is featuring a soundtrack that helps greatly to create the atmosphere of unknown terror on the screen - in 50s terms that is, of course.
In X the Unknown, we see a lot of the tropes that Sangster will employ again, later in his career, when he has a tighter grip on dialogue and character: the two boys out on a dare, in particular, is a classic Sangster plot device.
In X the Unknown it happens- the two guards left on the fissure have regaled us through the picture with their comedy stylings (and they might actually be funny, but their accents are a bit impenetrable to these Yank ears.
Even imitators of the form, like X the Unknown and Prince of Darkness exude an intelligence in a genre that too often substitutes flash and thunder for brains; they do not particularly challenge your intelligence, but they at least assume that you have some.