Director Neil Marshall, of 'Dog Soldiers' fame, has crafted another superior horror film. Again he pits a group of people against an unknown and largely unseen enemy. This time however, instead of a group of armed soldiers, we have a group of unarmed, female potholers.

The film opens with Sarah (Macdonald) and her husband and child on vacation during which both are killed in a tragic accident, leaving a grieving Sarah to rebuild her shattered life. She decides to go on a potholing trip in the Appalachian Mountains, in an attempt to get her life back on track.

As soon as we enter the cave system the tension is cranked up a couple of notches, when a collapsing tunnel blocks their way back. Juno (Mendoza), the leader of the group, then tells them that it is a previously unexplored cave system, so no one will know they are down there, should they get into any trouble.

The film seems to be shot using only available light from torches, flares or green glowsticks, a great cinematographer is evident, and the atmosphere created adds to the overall sense of forboding.

We are down there for the rest of the movie and as it becomes more claustrophobic, the air of menace is cranked up yet again, as the girls discover they are not alone in the dark. Marshall pays homage to one or two great horror films of the last twenty or so years and one scene is very reminiscent of Carrie'.

With shocks and jumps every few minutes and the amount of gore present, even the most jaded horror fan should be satisfied by this genuinely frightening movie. Take a pack of Pampers with you, you may need them.

My only quibble is the weak, and somewhat confusing ending, but it still remains a suspenseful masterclass in gory terror. Hollywood must surely sit up and take notice of Mr Marshall.