A group of cave explorers and divers go to an impressive, never-before-explored cave in Papua New Guinea armed with lots of adrenaline, fancy equipment and horrible dialogue. James Cameron has put his name on this procuction, which proudly boasts this affiliation as well as its 3D photography. This review is based on the flat version, however, so I must reserve the unlikely possibility that the 3D version glosses over all of the film's weaknesses.
Because there are quite a few of those, and they are not exactly subtle. Firstly, the characters are flat as Denmark, caused by the never fortunate combination of bad writing and bad acting. Poor Rhys Wakefield puts too much effort into his cliched character, and ends up making it far too big for his own good. And with Richard Roxburgh playing the kid's father in Sly Stallone mode (which is actually quite effective, if not exacly subtle), you realize early on that the success of Sanctum is down to whatever excitement and thrills the filmmakers are able to create from its typical disaster film setup.
And impressively, the film improves as it goes along. After a while, the tacky dialogue dries up (probably because there are increasingly fewer characters with increasingly less to brag about), and the race against time and air is both exhilarating and fascination in a primal sort of way. There is a decisive scene towards the end, in which two of the principal characters say goodbye, which confirms that these guys are not really from our civilized, modern society; they're from ancient times, battling it out in an ancient cave. And with this realization in mind, the film's initial problems dealing with people in modern situations seem almost irrelevant. Sanctum is exciting, not clever.