I was going to go into a long description of this film and its narrative, but Exclamation Mark has already done a terrific job and hit most of the points I would have brought up. So instead, I'd ask that you look at his review and then come back for a few of my thoughts on the film. Go ahead, I'll wait.
Back? Good review, no?
First, the filmmakers chose an interesting setting for the movie, mysterious in its own right. The Salton Sea started as a basin, a prehistoric leftover of the once world-covering ocean. The basin was once the site of mining and a small town. Both were submerged after floods at the beginning of the 20th century. (All this information gleaned from Wikipedia.) The movie, in the opening voice over, points out the prehistoric origins. The Sea is also a good setting because it sits amid a desert. We get a sense of the place's isolation and are reminded of the desert settings of "Them," the template for all these giant bug movies. (The map seen here is stolen from the Notes from the Road Web site.)
The prehistoric origins and the earthquake seem to be the origin of the monster. In fact, the scientist goes out of his way to dispute the notion that radioactivity tests had anything to do with it. And that's a strange point, because later we find out that the monster's saliva is significantly more radioactive than the surrounding water. Yet the point is never broached again. Was the monster revived thanks to radiation? Or did the movie makers decide that a giant monster just had to have radioactivity tied in somehow?
Exclamation Mark points out that the movie is slow, and I agree. It plods along with little forward drive. Even once we know that the hero must destroy the creatures before it finds its way out of the sea, the pace still moves leisurely. This and a need for more monster action really doom this film from repeat viewings. It's just not that entertaining.
But don't blame the actors for that. Lt. Commander John "Twill" Twillinger is made out to be a stickler for Navy regulations. He accepts no walking outside the lines, for whatever reason. Later, we get his softer side as he plays with Gail's child and shows his willingness to forgive errors in the field. It may not have helped viewers like the character, but the choice makes for a far more three-dimensional character than in many other giant monster movies.
And the same goes for almost all the other characters. From the archives guy obsessed with a defeated proposition to a gatekeeper shooing away kids, everybody has his own personality. It's too bad they weren't in a faster paced film.
The monster design wasn't bad. Though they say it's a snail at one point and it sure doesn't look like one. It's got hard skin, mandibles and little arms. It's a bug of some kind, not a snail.
We never see more than the top of the creature. It has a long cylindrical body and we have to assume it has a shell just out of sight, since later we see the creatures hiding in their shells.
The monster's eyes are probably its best asset. They make the monster look like something from the cover of an old pulp science fiction magazine. Science fiction and fantasy author Dave Duncan wrote the story behind the script. Here's a nice paragraph about the film from this Turner Classic Movies site:
The Monster That Challenged the World was shot in sixteen days on a budget of $200,000 and reportedly Holt suffered a broken arm during one of the film's action sequences. According to co-producer Arthur Gardner in Science Fiction Movie Stars and Horror Heroes by Tom Weaver, "The mollusk monster was conceived by us and executed by a very good special effects man named Augie Lohman. Augie went on from that picture to do many, many famous special effects films (Barbarella, 1968). The monster stood around ten feet high, and the exterior was made of fiberglass. All the movements were controlled by Augie and two assistants - it took three men to operate it. It worked with a series of air pressure values. I believe it cost around $15,000 to build, and weighed about 1,500 pounds."
This is one movie where I could see a modern remake actually being worthwhile. If they maintained the design, but gave it a mobility boost, kept the interesting characters and added better pacing, this could be a great fun film. With a modern budget, you could even play out the great warning the scientist offers: "Can you imagine an army of these things descending upon one of our cities?" Yes I can, and it would be marvelous.
Anyway, the film is certainly worth a watch. However, it's not something I'll be seeking out again.