If there's an undersea adventure with high-tech equipment, macho posturing, and lots of underwater photography, you know James Cameron must be swimming around the vicinity. Add the fact that Sanctum was released to theaters in 3-D, and it's clinched. Cameron served as executive producer to this crazy tale of a cave-diving expedition forced to improvise when a typhoon inundates their New Guinea location. (The film, shot in Australia, is allegedly based on a true event by co-screenwriter Andrew Wight, but it might be safe to conclude that the original incident was a jumping-off point for the high melodrama on display here.) A globetrotting billionaire (Ioan Gruffudd, of Fantastic Four) is underwriting this exploration of a hidden cave maze, which explains why he gets to bring his girlfriend (Alice Parkinson) along. As a measure of their thrill-seeking habits, we are told they met on an Everest climb. The cave-diving boss is a crusty old pro (Richard Roxburgh), who is rough on his underlings and even rougher on his teenage son (Rhys Wakefield); naturally, the cataclysm that follows will be an occasion for some extreme father-son fence mending. As cornball as these elements are, and as generally toneless as director Alister Grierson's ear is with the dialogue scenes, Sanctum does work up some bona fide thrills: the sheer power of water is unleashed at a few memorable spots, as is the panic of losing an oxygen tank at a crucial moment. It's also pretty brutal, with a steep body count and a few grotesque bits of bodily injury. It ought to be easy to dismiss Sanctum as a silly piece of boy's adventure, but--curse you, Cameron!--one must admit that the thing is awfully effective. --Robert Horton