The Descent is one of the most effective horror thrillers in many years.

The film concerns a group of women who share a passion for extreme sports who undertake a spelunking expedition into a dangerous, unmapped network of caves and caverns in a remote part of Appalachia.

Director Neil Marshall has layered the film with several sources of horror and suspense and the abiguity of the title's meaning reflects the many shades of terror that lay hidden within, open to many interpretations.

The most obvious reference in the title is to the literal descent into the pitch blackness of a cave so frought with geological dangers that it seems to have a malevolent nature of its own, but that is only the beginning of what The Descent is about.

It is also about a descent into the darkest, most primal emotions of desolation, fear and rage. It's about the evolutionary descent of a lost branch of homo sapiens into a race of vicious, mutated cavern-dwellers. Most of all, it's about one woman's descent into madness.

The story begins as a trio of friends are ending a whitewater rafting expedition. What seems like an idyllic life quickly endures a series of brutally shocking events, which after the passage of time, lead to a new gathering of friends for an adventure that promises a way to heal emotional wounds and rebuild a life.

The six women who meet for the descent are long-time friends with differing personalities and varying athletic skills. Juno is the planner of the adventure, a woman with the kind of ego that leads her to make her own rules. Holly is a wild-child with the greatest taste for thrills, whose greatest passion is for base-jumping. Beth is the quiet academic and Rebecca and Sam are sisters with a love of rock climbing. Lastly, there is Sarah, the emotionally damaged friend for whom the gathering of friends has been planned.

Once the descent into darkness begins, the fear and suspense are ratcheted up moment by moment. There's never been a movie in which the sensation of claustrophobia is so vividly evoked. The darkness of the movie theatre becomes an extension of the darkness onscreen, until the audience is as one with the women underground, sharing their increasing fear of the unknown, eventually lost and threatened by the fearful dangers of unforseen hazards.

The visual experience of the underground world constantly changes through the filmmakers' masterful use color cast by different sources of light: the red of a torch, the green of glow-sticks or the harsh white of flashlights.

The tension from the dangers of falling rock, sheer cliffs and suffocating entrapment alone would satisfy a viewer's hunger for thrills, but the nightmare hasn't really begun until Sarah catches a glimpse of something inhuman awaiting in the shadows.

What follows is a terrifying race to escape, with violent bloodletting and brutal death as the gauntlet to be forced in order to survive.

The flesh eating crawlers look exactly as one might expect such creatures to have evolved, and their appearance is unforgettably creepy.

The Descent's ending is sudden but satisfying. The conclusion has been altered slightly from the original, European version of the film, and it will be up to viewers who experience both to decide which they prefer. Both endings are dark, but in different ways.

Director Neil Marshall is truly the new Master of Horror.