A B movie in the best sense, Sanctum executive-produced by James Cameron is a lean, clearheaded 3-D thriller that knows exactly where it's headed from the moment its adrenaline-junkie explorers dive into the world's biggest cave in Papua, New Guinea. They think they're in for some extreme spelunking; we know they're about to get knocked off one by one as a storm turns the cave into a waterlogged death trap. The extraneous bickering between our teen hero (Rhys Wakefield) and his tyrannical dad (Richard Roxburgh) flops, but character development isn't the movie's strong point. Momentum is. As we go deeper into the cave, walls squeezing, water rising, the movie has a narrative pull as sure as gravity. B

