Ninth installment in the Dirk Pitt ocean-bottom salvage saga (Raise the Titanic, Cyclops, Treasure), with a dramatic upgrading in the writing. This time out, Cussler keeps a tight plot under a favoring wind and does not fill out his 416 pages with a surplus of subplots--though, to be sure, the story builds on Saturday-matinee cliffhangers and has the usual aircraft blueprints, as well as the Cussler clangor of underwater hardware, for bolting down fantasy. (A character barely picks up a telephone without our getting its specs, including holographics--we're into 1993--and distant speakers facing each other in 3-D.) The story: In 1945, a third plane carrying an atomic bomb to Japan is shot down and sinks off a Japanese island. The waterproof bomb lies down there for 50 years. In 1993, Dirk Pitt mines the sea-bottom with a colossal submersible tractor near the lost plane when a huge Japanese automobile-carrying cargo ship miles above him blows up, destroying two other ships nearby. It seems that a secret Japanese crime cartel, set on raising Japan to world trade dominance starting with a takeover of the US, has been making A-bombs. Lacking missiles, the cartel smuggles its small A-bombs in Japanese automobiles into various US cities and is now ready to blackmail the President for their big takeover. The cartel works out of Dragon Center, the island near where the US A-bomb sank. Dirk Pitt, now drawn into a US secret agency for locating the Japanese bombs (the cartel explodes one bomb in Wyoming for demonstration purposes), is given a new submersible tractor, since his last was destroyed in the accidental A-bomb explosion of the automobile cargo ship, and is sent down to blow up the US bomb in the sunken bomber, thus causing an earthquake and tsunami that will wipe out Dragon Center. Naturally, blowing up an A-bomb poses some threat to Dirk's life--but he does it, and his submersible sinks under a tremendous mudslide into a huge trench. Next we are reading deathproof Dirk's obit. Can he really be. . .? More surpassingly improbable than Indiana Jones, but much fun, crisply told, with exciting special effects. By now, Cussler has spent nearly 5,000 pages mucking around in oceanic blackness. Obsessive?