This movie isn't particularly well known (or at least well remembered) but is actually a reasonably good example of 50s monster sci-fi. The Monster That Challenged The World (MCW) fits somewhat into the classic "big bug" sub-genre by its situation and plot. The monsters are called giant mollusks, though amount to 20' monster aquatic caterpillar things. Just what they are doesn't matter too much, though. Like any good big-bug movie, they arise, terrorize and must be stopped. The monster is actually pretty well done for its day. The acting is reasonable too. Overall, this makes MCW a good watch for a 50s sci-fi fan.
Quick Plot Synopsis
Some mild earthquakes rumble through southern California. A naval lab tests parachutes with jumps over the inland salt lake, Salton Sea. A jumper disappears, as do the two sailors sent to pick him up. Their bodies are found, shriveled, drained of all fluids. A gooey slime on the boat is the only clue. The slime turns out to be mildly radioactive. Next, a young woman and her boyfriend are lost, swimming at night. The same slime is found. Divers look for the bodies and find a strange sack-like "egg" but also a giant creature in an underwater cave. One diver dies, the other escapes. The egg is taken back to the lab. Dr. Rogers (Hans Conried) theorizes that eggs from ancient sea mollusk (he calls the Kraken?), but the mildly radioactive waters of Salton Sea have mutated them into giants. They are voracious eaters and lay thousands of eggs. If they escape the lake into the canals, and thence to the ocean, they could threaten the whole world. Sure enough, they do get into the canals. The navy finds their 'nest' and blows them up. All would be well, except for the egg back in the lab. The secretary's daughter innocently turned up the holding tank's heat, so that egg hatches. It quickly grows and chases the secretary and girl into a back room. Just as it appears the beast will get them, the hero arrives and distracts it. Just as it appears the hero is about to be eaten, sailors arrive with guns and kill it. Hero and love interest are saved. The end.
Why is this movie fun?
For a low budget movie, MCW is pretty well done. The early movie attacks are kept suspenseful. The monster itself is well done for the pre-CGI era. MCW is a worthy addition to the big-bug family.
Cold War Angle
Other than the common source thread (radiation caused the monsters), the story is much more a monster tale than a metaphor for the Cold War. The prominent and heroic role of the American military was completely acceptable to 50s audiences.
Better Monster -- The animatronic giant "sea mollusk" is fairly convincing and reasonably well done. This monster stands in stark contrast to the bizarrely bad space bird in The Giant Claw, released around the same time. Having a huge real beast model helps the actors relate, much as having the giant ant models helped in Them! ('54). The giant mollusks are reasonably scary looking.
Special Locale -- The movie is set on southern California's Salton Sea. Part of the explanation of the monsters' origin is that they were ancient life mutated by mild radiation (bad ol' nuclear testing) and water seeping underground via new earthquake fissures. Salton Sea was a dry salt flat until 1905 when the Colorado River jumped its banks and flowed via an irrigation canal to flood the flats. Tiny dormant shrimp were revived from the last time the flats were flooded. This happened periodically over the centuries. This becomes the premise to explain the 'new' monsters.
Nuke Legacy -- Even though nuclear test explosions weren't done at the lake (the area is far too populated), the crew of the Enola Gay did do practice bomb runs over the lake. No "live" bombs, of course, but it's still a connection to the nuclear age.
Pre-Jaws -- One scene features two young lovers going for a night swim. First the man disappears. While the woman is treading water, frantically calling for him, she feels a tug at her leg. She screams and is pulled into the inky water. It's a very Jaws-like scene, but almost 15 years earlier.
Star Watch -- Tim Holt, as the cranky Commander, was a staple of B-westerns since the late 1930s. This was the last of his regular work movies. He did only a few bit roles afterward. Hans Conried, who normally plays comic roles, such as in The Twonky ('53), gets a serious role as the all-knowing, if a bit testy, scientist.
Bottom line? MCW is a pretty well done B monster movie from the later 50s. It follows the formula plot of Them! in several ways, but in a fresh enough form to not seem like a rehash. It's worth watching for monster movie fans, provided one isn't too jaded by modern CGI and expectations of salacious gore.