Here's the deal: Sanctum is about extreme people in extreme danger in an extreme place. In 3-D! The good kind of 3-D, the kind that immerses you in a place you've never been before, not the kind that makes the actors look like bobblehead dolls while turning your stomach in the process. You get to swim around an enjoyable approximation of a multidimensional representation of breathtaking subterranean realms the likes of which you'll never ever see with your own eyes. And both Ioan Gruffudd and Richard Roxburgh take their shirts off. What, you need Shakespeare, too, on top of all that?
Sanctum is cheesy. Yes, it is. It's like one of those old IMAX science documentaries you used to go to on school field trips, crossed with a deliciously overblown 1970s disaster movie, like maybe The Poseidon Adventure. It's pretty much the definition of a popcorn movie: it's visceral and not something you need to think too much about. There are moments of shocking intensity of the ultimate-survival, life-and-death kind, performed by archetypal characters who are sketched out in ways we've previously agreed upon: the cocky billionaire, the father and son who misunderstand each other because they're actually so alike, the overconfident adrenaline junkie. And so on. If you don't want to be generous, you might call them cardboard cartoon cutouts. If you want to be generous, you could call them almost operatic in their broad strokes.
You don't go into a movie like this expecting anything deep, except in the way-underground sense, of course. Though I doubt the Esa'ala Caves of Papua New Guinea, that we're introduced to here are actually the largest unexplored cave system in the world. The idea of such a place thrills me, and director Alister Grierson — whom you've never heard of since he's got one Australian film to his credit and a few shorts — presents this world to us in a beautifully thrilling way. He gets huge assists from producer James Cameron, through both the 3-D process Cameron developed for Avatar and the aquatic shooting techniques he honed on his lovely underwater documentaries Aliens of the Deep and Ghosts of the Abyss, and there's a sense that the team of divers exploring this cave, whom we meet as the film opens, are akin in their seriousness and dedication to their work as Cameron was for Aliens and The Abyss. These divers are people for whom technology is the difference between life and death, and the film has a solid respect for the cold equations of scientific reality when it comes to how to stay alive in such an unforgiving environment. One diver hasn't seen daylight in more than two weeks, we learn as the movie opens: She's been two kilometers underground that long. And that creates a kind of stress that's unique to her situation.
Staying alive is the crux of the story: Billionaire backer (Gruffudd) and his Everest-climbing girlfriend (Alice Parkinson) head down two kilometers for a visit to his investment. Their teammates include a veteran lead diver (Roxburgh) and his teenage son (Rhys Wakefield), who he has a contentious relationship with. Soon, they're cut off from their only known exit by flash floods. The only way out now is in — and into territory they haven't finished mapping. There has to be a way out; the underground river they're following has to empty out somewhere. But they have no idea where it is, or if their now-limited equipment will help them reach it.
There are gorgeous yawning caverns here, and claustrophobic tight spots; squeezing through them with essential breathing gear is nail-bitingly excruciating. Even this rugged equipment is fragile next to the mountains of sharp rock pressing down on the characters in 3-D, and not everyone makes it out alive. In fact, the film, in its opening moments, makes us question whether anyone will make it out alive at all. And the dangers faced along the way are more than enough to convince us that survival must surely be impossible. Though it is only loosely based on an actual experience by undersea explorer Andrew Wight (also a producer on Cameron's aforementioned documentaries), it does have an unclichéd ring of truth in its casual lethality. And that makes for a much more gripping cinematic experience than the cheesiness would imply.