The Secret of the Caves (Hardy Boys, Book 7) Hardcover – January 1, 1992 by Franklin W. Dixon
Book 5 of 115: The Hardy Boys
 
Triggered by the disappearance of a brilliant young professor, this mystery takes the Hardy boys first to Kenworthy College, where they find a puzzling message on an examination paper. But then, a new clue is unearthed that sends the young detectives to the Honeycomb Caves. After many dangerous encounters,  Frank and Joe discover a powerful searchlight in one of the craggy caverns along the seacoast. In the brilliant white glare of the searchlight, the startling secret of the caves is dramatically revealed.
 
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