"The Descent" is the most relentlessly, rivetingly, ruthlessly horrifying film of the year. It plays every emotional key of fright and despair with the skill of a concertmaster at a cathedral organ.

From the very opening — a bombshell that will not be revealed here — Neil Marshall's film prods us from grief to claustrophobia, from fear of fangs to paranoid mistrust. The story is about a six-woman team of cave explorers negotiating a dangerous underground cavern system, but the title really describes the audience's free fall into terror.

The group is a collection of British and American adventure sports enthusiasts reuniting on the anniversary of a tragedy that befell a member on their last outing. Juno (Natalie Mendoza) has invited her friends to Appalachia for a caving expedition that is part reunion, part therapy for the bereaved Sarah (Shauna MacDonald). The women are a diverse, well-characterized bunch, from calm and collected medical student Sam (MyAnna Buring) to Holly (Nora-Jane Noone), a rebellious adrenaline junkie, and they enter the cave mouth in high spirits.

As they leave daylight behind and shimmy through narrow crevices, however, an ominous mood is quickly established, with Marshall conveying every tense sensation in atmospheric visual terms. After several serious mishaps, rational calm begins to fray. Their ordeal becomes a fierce fight for survival when they discover the cave is the lair of a decadent race of subhuman flesh-eaters — and that some in their own party would sacrifice others to escape. The film becomes both a violent horror movie and a sinister psychological thriller.

Marshall borrows ideas from famous horror movies and cult classics — it's impossible to miss the "Carrie" references — but he realigns them ingeniously. Monsters in a cave ought to be an unabsolvably hokey premise, yet the film never loses its uneasy and menacing air.

Though it's set miles underground, "The Descent" is stunningly art-directed and lit, with head lamps, colored flares and luminescent glow-sticks giving the action an eerie illumination. Several key sequences take place in deep darkness, and one caver's use of her video camera's low-light mode to peer through the murk is certain to give audiences a jolt.

Marshall made a brilliant debut a couple of years ago with "Dog Soldiers," a driving shocker about British commandoes under siege by werewolves. There's no question that he can still deliver shocks, as "The Descent" proves from its shattering prelude to its perverse denouement.

But it's the chilling transformation of the characters' personalities as they battle their way back to daylight that marks this as an important step in Marshall's evolution as a storyteller. He could leave an indelible imprint on the horror genre one day. Perhaps he already has.