Under the Sea3 books in series
Secret Under the Sea Publisher's Summary
Why is his dolphin acting so strangely? Something must be wrong. It is the year 2013, and Robby lives in an Underwater Research Station with his scientist parents. Most of the time he has fun exploring the ocean caves with the dolphin who is his favorite companion. But something has frightened the dolphin, and Robby sets out to investigate. Then he finds the giant footprints. And he knows that something enormous and unknown is walking here across the bottom of the sea.©1960 Gordon R. Dickson (P)2014 Audible Inc.