“Sanctum” begins, “Inspired by a true story.” That story was the 1988 Nullarbor cave expedition that had 13 people trapped underground. Unlike the high-tech equipment used by the characters in the film, the Nullarbor cave-divers trapped 80 metres underground had to make do with radio communications. They had gone into the cave to make a documentary and were trapped by a rock slide. Fifteen explorers were trapped for two days, including Andrew Wright, a producer of “Sanctum” and a documentary about the incident called “Nullarbor Dreaming.” Fortunately, the real-life explorers had a better outcome than the movie’s characters.