Director Neil Marshall's subterranean nightmare traps six women inside an unexplored cave system, where physical danger soon gives way to primal terror. The film methodically strips away safety, light, and certainty, forcing it's characters-and the audience-into a test of endurance. Anchored by Shauna Macdonald's emotionally scarred survivor, the story unfolds as both monster movie and psychological ordeal. Released in 2005, it is one of the most effective survival-horror films of it's era.