Twenty-one separate cave sets were built for the film. These were carefully reused with different camera angles, set dressing and lighting to suggest a nearly endless collection of interconnected tunnels and caverns. For realism, the makers often limited the lighting of the sets to light sources that the protagonists brought with them, such as flashlights, helmet lights and light sticks.
The scene where Sarah finds Beth barely clinging to life originally featured some "truly shit" dialogue, and the actors let that be known. Neil Marshall agreed, and the three went to a nearby pub the night before filming and rewrote the dialogue on a napkin. The film's producer chewed Marshall out for it, but he also agreed the scene was vastly improved now.
The filmmakers considered it too dangerous to film in an actual cave. It also would have been far too time-consuming, so they opted to build one instead.
Neil Marshall shot many of the crawlers at 18 frames per second with a 45-degree shutter to give them a "staccato movement."
According to the audio commentary on the special edition DVD, there was an exact replica made of Nora-Jane Noone and after the movie was finished, she got to keep the head of the doll. Once, she put it in her mother's freezer as a joke.