The Unknown Terror is a 1957 widescreen American horror science fiction film directed by Charles Marquis Warren and starring John Howard, Mala Powers, Paul Richards and May Wynn.

The narrative follows a group of explorers who, while searching for a missing man, come across the "Cave of the Dead", filled with parasitic fungi and inhabited by foamy, fungus-covered monster men.

Sir Lancelot, the King of the Calypso, performs a song with cryptic lyrics that Dan believes refer to the Cave of the Dead: Down, down, down in the bottomless cave/Down, down, down beyond the last grave/If he's got the stuff of fame/If he's worthy of his name/He may get another chance but he's never more the same/He's got to suffer to be born again.

The situation becomes tense when Concha takes Pete and Dan to a place where they can hear the voices of the dead crying from beneath the earth.

Pete and Gina discover that the cave walls are thick with a fast-growing parasitic fungus, the same stuff that grows on Ramsey's canned fruit.

Pete and Gina don the diving gear they've brought along and swim from the cave to the safety of a beautiful tropical beach.

[11][12] The movie was not available for individual home viewing until 2004, when Teakwood Studios put on sale an "unauthorized" release in VHS and DVD formats.

"[14] BoxOffice offered readers only a brief synopsis of the plot, which noted that Gina and Pete "escape to bring to fruition the love they had always had for each other."

The magazine suggested in its "Exploitips" that exhibitors create public interest by having "an aid" from a newspaper, TV or radio station watch the film with an audience and arranging "with a local doctor or psychology teacher to test the person afterward for pulse rate, effects, etc."

"[19] Bryan Senn, the American film critic, says in a similar vein that "it's difficult to make a bunch of advancing soapsuds look menacing" and that "the creatures themselves are never shown too clearly, which is just as well since they inspire more menace in spongy silhouette than in full bubbly view (upon which they look just like what they are – actors with bits of dirty cotton stuck to their faces).

"[18] British film critic Phil Hardy writes that "the monsters are unintentionally hilarious – they are simply actors entirely covered in soap suds.

"[17] And Warren says he "wonders hopelessly why the filmmakers didn't consider how audiences might react when they saw huge quantities of suds pouring down cave walls.

"[3] And while Howard called the monsters "just nonsense" in an interview, he also said that "in viewing the movie, I thought that it looked like soap bubbles coming down the cave walls.

He writes that the songs performed by Sir Lancelot "are well used as purveyors of taboo folk wisdom, with one line, 'He's got to suffer to be born again,' vividly foreshadowing the ensuing religious emphasis on the film."

As an example, Craig writes that when he's asked about the Cave of the Dead's precise location, "Ramsey chuckles, 'I can show you how to find that – it's their purgatory ...' as he points his spoon towards the flames under his boiling pot.

A more literal reference to the fiery flames of the Christian Hell cannot be imagined and the audience now knows that what the natives fear, and the white men seek, is access to death.

"[21] Laurence Raw, a British film scholar, makes special note of the role of Sir Lancelot in The Unknown Terror.

To Raw, Sir Lancelot's importance is that he embodies "mainstream Euro representations of the African American experience – on the one hand he portrays the Caribbean as a holiday-maker's paradise, full of sunshine, sea and limitless pleasures; on the other hand it is also a place of great danger, wherein westerners are perpetually vulnerable to attack by unknown forces.

The exhibit featured "rare photographs and promotional graphics [which] are used to trace calypso in phonograph recordings, song publishing, nightclub acts, concerts, Broadway shows and Hollywood movies."