The ballistics-missile submarine Jules Verne lies 8000 feet beneath the Pacific when suddenly it is attacked and punctured by. . . Megalodon, a 90-foot prehistoric shark with monstrous teeth (one of which is left behind as evidence of the creature's identity). And over at the Institute of Marine Studies in Hawaii, director Frank Acremann hopes to establish the truth about Megalodon by using the stars of his dolphin-communication research. So he flies his two dolphins, Doris and Macho, and an immature Orca whale, Morgan, to the Molokai Fracture Zone, the ocean's deepest trench. But fellow marine-biologist Harry Asquith suggests that their animals can't go deep enough for a real investigation--and that a trained sperm whale might go down at least 5000 feet. Thus, while the hungry, huge-mouthed megalodons (parents and a baby) graze on ship garbage and attack another sub, the biologists go north, trap a whale (""Apollo"") into a sling, and carry it hung from a dirigible back to the Zone. Doris, Macho, and Morgan play with Apollo and explain to him what Acremann wants: sonar pictures of the megalodons. So eventually Apollo battles the megalodon youngster and escapes. . . while a torpedo wounds the shark, causing the parents to pounce on him in a feeding frenzy and chomp up a nearby Russian sub as well. And, when last seen, Acremann's animal trio is escorting Apollo home to the Arctic. . . . Very slim on fiction values--but readers with an interest in marine biology may find the dolphin and megalodon lore intriguing.