The Secret of the Silver Dolphin — Carolyn Keene

A newscast announcing a reward to anyone who can locate a valuable silver dolphin involves Louise and Jean Dana in a baffling search. Judy Platt is desperately trying to find the silver dolphin mentioned in her dead brother's will. But no one knows what kind of dolphin to look for -- a live mammal or a silver object.

The Danas' sleuthing is hindered by a hostile fortune-teller and an elusive thief. The two are engaged in a sinister conspiracy to get the reward -- and will stop at nothing to ensure the success of their criminal scheme.

Louise and Jean's quest for the silver dolphin leads them to Miami, Florida, and then to a deserted Caribbean island. A dangerous skin-diving chase, followed by an amazing undersea discovery, results in the Danas solving this intriguing mystery.