Twenty minutes into The Descent, in a moment of genuinely claustrophobic terror, a traumatised young woman trapped within an entombing body of rock is told to relax because 'the worst thing that can happen to you has already happened'. This turns out to be, at best, a grim half-truth, although it's hard to imagine anything matching the emotional sucker-punch of the film's gasp-inducing opening sideswipe. Yet even as shocking bereavement gives way to more generic dismemberment, director Neil Marshall's nihilistic frightener keeps us believing that things can only get worse. Thus we plunge down the dark, dank pothole of one of the best British horror films of recent years.

On the anniversary of a fateful white-water rafting trip, a trio of independently spirited women reconvene in the Appalachian Mountains, part of a six-strong girl group determined to pit their wits against the area's labyrinthine caves.

The group dynamics are strained from the outset, with Sarah (Shauna Macdonald) still heavy-laden with loss from the previous year's accident, and Juno (Natalie Mendoza) harbouring a guilty secret that Beth (Alex Reid) already suspects. Pressure mounts when the women lower themselves into a cavernous crater within whose darkness lies the promise of hallucinations, paranoia and asphyxiation. When a rock fall cuts off their escape route, the group discover that 'the only way out is down', venturing further into the bowels of the earth, squeezing themselves through ever-decreasing circles as they journey toward a seventh level of hell.

For a good 40 minutes, Marshall, who cut his teeth on the rough-and-ready squaddies versus werewolves romp Dog Soldiers, grips his audience firmly by the throat, squeezing the breath out of them with a bravura evocation of the real-life terrors of entrapment. Whether scrabbling and scratching through ominously dripping tunnels or swinging precipitously across vertiginous chasms, this rag-tag band endures palpable physical endangerment that leaves the viewer gasping for air.

Then, somewhere around the halfway point, Marshall loosens his suffocating neck lock and switches to body blows as The Descent downshifts from down-to-earth anxieties to full-on fleshy fantasy. It's a deal made with the devil that effectively sells out the film's human soul, but which, in return, reaps the crowd-pleasing rewards of slimy monsters, big noisy shocks and enough meaty scrunge to feed an army of cinematic cannibals.

Proving once again that horror is a genre that feasts upon its own entrails, The Descent duly devours and regurgitates an impressive smorgasbord of shockers. Marshall describes his movie as 'Deliverance goes underground', but this feisty thriller owes just as much to the town-and-country counterpoint of Wes Craven's The Hills Have Eyes. From the low-angled shots of an Evil Dead-style woodland cabin, to the iconic poses of Carrie (the blood-splattered Macdonald bears more than a passing resemblance to Sissy Spacek), Marshall leads us on a whistlestop tour of all his fan-boy favourites.

Echoes of Nosferatu, Aliens and even Danny Boyle's 28 Days Later reverberate around these caves, with a feast of Fulci-style gore laid on for the hardcore horror fans. Breaking with the stalk-and-slash tradition of a single 'final girl' surviving to slay the beast, Marshall presents an ensemble chorus who turn upon one another as they descend into the mouth of madness. In stark contrast to the male-bonding of Dog Soldiers, the heroines of The Descent, which Marshall calls its 'sister film', become every bit as deadly as their savage surroundings.

Seamlessly blending surface location footage with Simon Bowles's beautifully designed cave sets (spiced up with some very effective matte and miniature work), Marshall and cinematographer Sam McCurdy conjure a world of subterranean darkness, illuminated by the unearthly glow of red and green flares, pierced by the dwindling torch beams of the climbers' helmet lights.

In one particularly effective sequence, the infrared setting on a handheld video camera is used to cut through the blackness, with startlingly chilling results. Elsewhere, an effectively unsettling sound design fills in the visual blanks, while David Julyan's harrumphing score moves from the lush pastoral strains of the outdoor scenes to the bellowing underground alarums and honking horns of hell.

For all it's knuckle-biting success, there are deep faultlines in the rock face of The Descent. The careful character development of the early scenes is lost in the film's frenetic third act, a situation worsened by the fact that two key characters look disorientatingly similar