A team of government scientists has just returned from a five-day expedition to explore shipwrecks in a mysterious part of the planet - the bottom of the ocean off San Francisco and Marin counties. This week's expedition is the beginning of a two-year archaeological survey of the Gulf of the Farallones, more than 1,000 square miles of the Pacific just west of San Francisco. "We know more about the surface of the moon than we do about the bottom of the ocean off San Francisco," said James Delgado, director of the National Oceanic and Atmospheric Administration's national heritage program and a project leader.