Piers Anthony - Mer-Cycle

A discreet advertisement brings a group of apparently disparate individuals together to a bizarre rendezvous – on the ocean floor.The reasons for their selection are unclear: Don, an archaeologist, chronically shy, Gaspar, marine biologist, suffering from terminal directness, middle-aged Pacifica, and Melanie, whose normal exterior masks a strange genetic inheritance, seem, on the face of it, to have little in common – except their feeling that they are part of a greater plan … a feeling that grows as they embark on their strange odyssey across the bed of the ocean …For the underwater explorers, the mystery of being out of phase with the world above water is heightened by that surrounding the mysterious Eleph – a mystery which is ultimately revealed to be more significant and bewildering than they could ever have imagined …