Cachalot

A guilt-ridden Earth had turned Cachalot over to the few surviving cetaceans as a perpetual refuge - a planet whose surface was one great ocean, where the remnants of the whales, porpoises, and dolphins could pursue their lives and perhaps even the development of an intelligence greater than man's.

Humans on Cachalot were stricktly confined to a few floating islands and the floating towns, prospering from wealth of its sea. The cetaceans seemed to have forgiven the thousands of years or terror and slaughter they suffered - some had even befriended sekected humans.

But something was destroyig the towns of Cachalot - leaving no clues... and no survivors.