Neil Marshall broke into the big time with his 2005 movie, The Descent. I had been a fan since his earlier werewolf film, Dog Soldiers. The two have a lot in common and I am a big fan of the Descent.
Dog Soldiers featured a pretty much all male cast, going to do one thing in a remote location and ending up trapped in something terrifying. The Descent alters that formula only by gender as a group of female friends go spelunking only to find they are not alone in the caves.
Like all good horror, a sense of claustrophobia really makes this movie work. Placing these four women far away from daylight, open spaces, help and rescue really ups the danger factor for those involved. Being isolated is bad enough, but sometimes having to squeeze through tiny spaces between massive rocks is extra nerve-wracking. In fact, this movie would be intense enough even without the introduction of the mutant cave dwellers who are all to eager for some fresh flesh to consume.
That same key to the horror, the cave setting, is the only way the women can fight back. Instead of being helpless victims picked off one by one, they have to use their underground skills to make quick escapes or fight back. This horror satisfies on the level of both being terrorized and the catharsis of turning the tide on one's attackers. Depending on which ending you see, you will get a very different emotional outcome. I first saw the European ending, which is very moving. The American one is a little more brash and "in your face." Both leave the protagonists in a very different place so watch carefully and make your own decisions about which was better.