Secret Warning — Franklin W. Dixon

Strange events involve Frank and Joe Hardy in a mystery which shrouds an ancient treasure -- the golden head of the Pharaoh Rhamaton IV. First, a cryptic unsigned warning from Egypt; second, the ghost of a bloodthirsty pirate of the eighteenth century, who, according to legend, haunted nearby Whalebone Island years ago and recently has reappeared.

The owner of the million-dollar golden Pharaoh's head claims it was aboard the freighter Katawa, which sank not far from Whalebone Island. But suspicious developments indicate that Mehmet Zufar may be trying to defraud the shipping line's insurance company. Frank and Joe enthusiastically accept the challenge of their famous detective father to assist him in investigating the complex case for Transmarine Underwriters.

A puzzling clue leads the Hardys to Whalebone Island, where they almost lose their lives in a violent explosion. Someone desperately wants to get rid of them. But who? Is it the ghostly pirate?

Frank and Joe's perilous scuba-diving search in the ocean depths off the island yields electrifying discoveries that cap the climax of this exciting mystery.