It's questionable that this movie qualifies as sci-fi, though it does get listed as one. It fits the horror genre better. The title and poster suggest something sensational, but the situation and plot are fairly formulaic B-movie material. A mad scientist is doing horrible experiments on innocent villagers in a remote and exotic location. There is actually very little "science" offered beyond a bit of exposition from the antagonist about the amazing fungus he's discovered in the jungle. The characters are fairly stock as well. There's a rich American explorer, his beautiful wife, a former flame fellow explorer (just for the love triangle flavor), a mad doctor and trusty stoic natives. They all come together because the rich explorer is looking for his wife's brother who disappeared looking for the "Cave of Death."
Quick Plot Synopsis
An explorer is lost in a Central American cave. He looks terrified before the fade-to-black. 6 months later, a wealthy explorer (Dan) says he has a new clue to his brother-in-law's disappearance. It's a song that came from a particular village. A (real) calypso performer sings a downbeat ballad of "beneath the grave" and "must suffer to be born again." Dan flew in a villager to interpret the song, but Raoul clams up. Undaunted, Dan, his wife Gina and a friend with a bad leg, set off on the expedition anyway. Once in the village, Raoul mysteriously disappears. They are directed to an American doctor living at the edge of the village. Dr. Ramsey denies the existence of any "cave of death". He tells of his work with jungle bacteria and fungii as research for antibiotics. He also tells how the villagers periodically sacrifice one of their own to the god of death, but he (as god of life) claims them and takes them to a "safe place". Ramsey's native helper, Lino, takes Dan and Pete to the cave. Inside, they find skeletons and Raoul's body. Going further, they find missing-Jim's shovel. Dan enters a small passage and screams. Some mutant thing clubs him. The cavern is quickly flooding, so Pete has to flee. He returns with Gina, Lino and some gear. When Pete and Gina enter the small passage, Lino blows it up with dynamite (killing himself too). Pete and Gina find Dan with a broken back. Oozing "fungus" comes down the walls. Pete seeks an alternate exit and finds a door to Ramsey's house. The fungus cannot be stopped, so Pete orders the cave blown up and sealed in. Dan has since died, so Pete and Gina don scuba gear and swim out through a cave pool. They walk arm in arm out of a sea cave. The end.
Why is this movie fun?
In a sort of foretaste of The Blob, the "monster" here amounts to streaming masses of soap suds. The actors react as if the suds were toxic. This is unusual enough to be interesting.
Cold War Angle
As a typical monster movie, there are no Cold War metaphors discernible.
Typical Mad Scientist -- Dr. Ramsey is the stereotypic mad scientist. He has an overactive ego ("I am the god of life to these people") and likes to play God with his experiments on his super fungus. Unlike most movie mad scientists, Ramsey never explains why he subjects the villagers to the fungus (which clearly mutates them), nor what he expects to do with his super fungus. Ramsey does die, however, like most mad scientists do, amid his creation.
Atypical Hero -- Pete is interesting as the hero for being atypical. He's the usual rugged handsome type, but is given a bum leg. He is similarly given a conflicted personality -- haunted by the past and doubting himself in the present. While all the other characters are two dimensional standardized B-movie types, Pete is interestingly complex.
Woman's Place -- An odd feature to Unknown Terror is how women are portrayed. Gina is the curvaceous glamor wife (even in the jungle, she's in an elaborate formal nightgown.) Ramsey whips his native "wife", Concha, because she dropped one of his fungus-growing jars. The others protest, but she says it's okay, she deserved it. (?) Gina is pretty much useless on the expedition, except as a frail (if desirable) female to be menaced by monsters and rescued by the hero.
Semi-Zombies -- Despite the promise in the movie posters, the infected natives play a fairly small role. From the brief glimpse you get, they appear mutated to some degree. There are several of them in the cave. A few get out to roam the jungle (and peek in Gina's window), but otherwise seem pointless. The fungus suds "eat" them as well as regular humans, so their role is ambiguous. They attack Pete and Gina (and we assume Dan), but for no apparent reason other than that it's just what zombies do.
Muerte II -- This movie is reminiscent of Mesa of Lost Women ('53) in having a mad scientist in a remote (and Mexican) location, preying on frightened local villagers. Ramsey, like Dr. Aranya, mutates the locals. Dr. Aranya's mesa was in the Muerte Desert (Desert of Death). Ramsey's fungus lives in the Cuevo de la Muerte, (Cave of Death). For B-movie writers, the (something)-of-Death was a handy formula for a place name.
Bottom line? As a 50s B-grade horror film, Unknown Terror is average. Sci-fi fans will find no aliens, no saucers, no electronic gizmos, etc., and very little science. Those caveats aside, it is a passable B movie.