The third collaboration of writer-director James Cameron and producer Gale Anne Hurd (Aliens, the Terminator movies) is a big-budget action thriller about a group of underwater oil diggers who go looking for a lost nuclear submarine and wind up encountering extraterrestrials. Shot largely underwater and with direct sound, this has a visceral kick to it that enhances Cameron's flair for high-tech special effects and streamlined storytelling, but the attempt to extract the essences of several genres (cold-war submarine thriller, love story, Disney fantasy, pseudomystical SF in the Spielberg mode) and mix them together ultimately leads to giddy incoherence. Before the movie collapses, however, there are several highly effective suspense sequences, and Mary Elizabeth Mastrantonio is especially fine as the feisty, volatile heroine. With Ed Harris, Michael Biehn, Todd Graff, and John Bedford Lloyd. 140 min.