Much of “Below” is spent keeping the characters in the dark, often literally, about what’s going on. This is an admirable manifestation of the law in movies that the less we see of the monster/ghost/maniac, the scarier it is.

Eventually, however, you do need to show us something. “Below” procrastinates that moment for too long, and when we finally do discover what’s happening, the answer is rather disappointing. It’s a case of the symptoms — the weird noises, the creepy phenomena — being more interesting than the disease.

Those symptoms, though, are fantastically creepy, and “Below” is an excellent ride until its finale. It is set in 1943 on a submarine — an ideally claustrophobic place for a horror film, you’ll agree; there is literally no place anyone can run to, and director David Twohy (“Pitch Black”) exploits that fully. The tight corridors and small round windows, the eerie silence outside, the lack of contact with the rest of the world — if there’s a killer among you, this is the last place you want to be.

The American crew of this sub picks up the three survivors of a hospital ship that was attacked, unprovoked. They are a German, a British man and a British nurse, Claire (Olivia Williams), who doesn’t divulge the German’s Germanness until it is too late and his actions become suspect.

We slowly learn of what has previously taken place aboard the U.S.S. Tiger Shark. It has only recently come under the stern leadership of the weathered Lt. Brice (Bruce Greenwood), the previous skipper having died in an accident. Capt. O’Dell (Matthew Davis), fresh-faced and eager, doesn’t trust the new commander, nor agree with most of his actions regarding the new arrivals.

But the below-deck politics are secondary to the phenomena that begin to occur, involving ethereal voices and ghostly images. Something or someone appears to be targeting the sub; the question is whether the forces at work are supernatural or caused by man — and, more to the point, what do they WANT?

The uncertainty of what we’re looking for gives the film a great advantage. In most horror films, we know approximately what the villain is — at least whether it’s a ghost, a person or a monster — but here we are struck with the dark realization that it could be ANYTHING. (I am particularly impressed with one hair-raising scene involving a mirror and a transient reflection.) Some seamen repairing an outer part of the submarine are harassed by a school of barracudas — irrelevant, maybe, but valuable in establishing the general dangers that face these men even without deliberate mayhem being caused by unseen forces.

The acting is heightened, with characters barking a lot of things at each other without a lot of regard for nuance. The dialogue, by Twohy, Darren Aronofsky and newcomer Lucas Sussman, is not as good as it ought to be, given Aronofsky’s resume (“Requiem for a Dream”). But maybe it does make sense, given Twohy’s past work (“Waterworld,” “G.I. Jane”).

At any rate, the atmosphere is effectively tense and forbidding for most of the film. The ending may not satisfy you, but you’ll have enjoyed most of the proceedings to that point. And if by chance you work on a submarine, you will probably have a hard time reporting to work Monday.

C+ (1 hr., 35 min.; R, some harsh profanity, very brief partial nudity, a lot of grisly images.)