The movie features the element of a mystery that was likely part of the true story. In the real events, there was probably something that people couldn't explain, and the movie turns this into a full - blown mystery about what's below. The behavior of the characters in the face of the unknown in the movie might be based on how people in the real story reacted to the strange events. For example, if people in the true story were hesitant to explore certain areas, the characters in the movie show similar hesitancy. Also, any signs or symbols that are associated with the devil in the movie could have been inspired by things that were seen or rumored in the true story.