BY SCOTT MACAULAY

There are monsters in Neil Marshall’s The Descent — icky humanoids with murky green glares and glistening gray skin — but there’s another reason why the film is the scariest (and best) horror movie to come along in quite some time. Simply, the film is terrifying before the monsters even enter the picture. In fact, when these Lovecraftian creatures (which the Lionsgate publicity team are trendily positioning as harbingers of global warming!) first make their split-second introduction, I was disappointed, not because their design was poor (it’s not) but because I didn’t think the movie needed monsters. Its tale of six female friends turning on one another while spelunking, surely one of the most visually frightening sports, had me on the edge of my seat the moment the flashlights starting dying, the crawl spaces narrowed, and the oxygen supply grew more distant.

Commenting on his initial concept, Marshall one-lines the film as “Deliverance goes underground.” “Instead of six men,” he says, “there are six women trapped and facing a common foe. But rather than bond together in the face of adversity, the women turn against each other, and their relationships disintegrate. It’s about a descent into madness.” The Descent tells the story of a group of women who, after one loses her husband and daughter in a car accident, decide to reconnect with each other by going on a caving expedition. Early on, however, it’s revealed that the group’s reckless leader has lied to them: the cave they are diving is completely uncharted, and what starts off as an innocent afternoon becomes a nightmare.

What’s great about the film is that it begins relatively naturalistically, placing us in the realistic world of spelunking and mapping the rivalries and resentments between the old friends with economical precision. As the film progresses — and as the women are hunted off by the monsters — the film becomes more and more stylized. By the end, the screen is bathed in saturated blood reds and night-vision greens, and the last woman standing, Sarah, played by Shauna Macdonald, morphs from a traumatized woman-in-recovery into a Ripleyesque action heroine.

Of the “crawlers,” McDonald says, “If a Klingon and Spock had a child, but it was totally hairless, and a bit shorter and quite sinewy, and was cousins with Gollum, that’s basically what a crawler would look like.” “We tried to give all the crawlers a distinct personality,” says prosthetics supervisor Paul Hyett. “There’s one called Scar who has lip pieces that pull down the corner of his mouth, almost like a dog snarling. We gave the females a witchlike look. We also did a child crawler with a bulbous head. All of them have little quirks.” The monsters took three and a half hours to build, with prosthetics, contact lenses and dentures going onto the actors — not dancers or stuntmen — who played the creatures. “I wanted actors rather than dancers to play the crawlers,” says the Marshall. “The makeup they have allows them to express themselves fully, both physically and with facial expressions, and putting actors in the makeup and costumes was ideal.”

The Descent opens in August.