We are told right from the start that this is inspired by a true story. But we quickly discover that it's not inspired by true characters. It's not surprising that a survival film like this would be populated with stock characters, but there is nothing surprising about anything that happens with them. The filmmakers wanted to make an underwater cave story and that is the only part he gets right.
Frank McGuire (Richard Roxburgh, MOULIN ROGUE!) is the best cave explorer in the world. He's a cold, no-nonsense taskmaster. His son Josh (Rhys Wakefield, TV's HOME AND AWAY) hates him for it and slacks off on his responsibilities at the latest expedition into a massive cave system that stretches miles into the Earth. The billionaire funder/adventurer Carl Hurley (Ioan Gruffudd, FANTASTIC FOUR) arrives to check out the latest discoveries. He has brought his new girlfriend Victoria (Alice Parkinson, X-MEN ORIGINS: WOLVERINE), who he met on a climb of Everest. While down in the cave, a freak storm hits and quickly begins to flood the caverns. The crew must follow the water down and hopefully discover its exit to the sea in order to survive.
Writer Andrew Wight is a diver who worked with James Cameron on such underwater docs as GHOSTS OF THE ABYSS and ALIENS OF THE DEEP. With John Garvin, he based the film on his own expedition that went array. The details of the external conflict are solid. Director Alister Grierson (KOKODA) and cinematographer Jules O'Loughlin capture the haunting beauty of underwater caves. The problem with the film is that none of these harrowing events are given any real human emotion or tension. The film doesn't come close to capturing the same claustrophobic terror that the spelunking horror flick THE DESCENT did.
All the characters can be boiled down to a few characteristics and catch phrases. Stock characters can be used compellingly when given strong motivations and compelling conflicts between each other. In cases of survival stories like this one, I always think back to the original POSEIDON ADVENTURE where the external plot existed to bring out the more compelling internal conflict between characters. This film beats us over the head with the father and son conflict and then throws in melodramatic personality conflicts right before the third act. Each of the characters dies in the exact order that we expect and none of the deaths raise the stakes in one bit. The cave is the cold villain, but it's robbed of its ominous threat right when it should be at its most scary as the film flips villains.
Frank and Josh's relationship drives the only dose of emotion the film can muster, but ultimately Josh is too weak of a central character. He doesn't care about caves and doesn't want to be there. Because he's established no more than the resentful son of Frank, he can't stand on his own. We don't truly care about his survival because we don't have a reason to really care about him.
SANCTUM is like a scuba lesson at the Y pool. There is a possibility of drowning, but there really isn't any true threat. The teacher stresses how life or death the information is, but there in class there is no tension, because you know what is coming next because you've read the syllabus before.