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Deep Shadow by Randy Wayne White (Doc Ford Series, Book 17)
In a remote Florida lake, a cave collapses, trapping Doc Ford and two of his friends. Ford manages to escape and surfaces to find help-but two ex-cons are waiting for him. They're intent on diving to the bottom of the deep lake and finding the remains of a legendary plane, supposedly loaded with gold. Ford's expertise is just what they need. And if he doesn't help, Ford and his friends are dead in the water.