Star Seed — D.C. Poyer

Years after the mysterious Great Dying kills every animal that breathes the air, only ten people remain alive, trapped beneath the Caribbean. Three adult scientists, studying the dolphins also confined at Navidad Bank Undersea Research Station; two college-age interns; and a strangely silent girl and boy, genetically engineered to live permanently beneath the sea. Joined by the crew of USS Pilgrim, a nuclear submarine returned from a secret patrol, they've eked out a precarious existence so far. But suddenly one day a new threat arrives; deep-sea sharks that act cooperatively and display abnormal intelligence. Commander Darrin Dinker Cross decides to trace both mysteries to their source. Who or what triggered the Great Dying, and continues to remold Earth's atmosphere, sea, and creatures? To find out, he accepts one final mission: a dangerous voyage far to the south. . . .