The Secret of the Caves (Hardy Boys, Book 7)

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.

• The Mystery of Cabin Island (Hardy Boys, Book 8)
• The Shore Road Mystery (Hardy Boys #6)
• The Great Airport Mystery (Hardy Boys, Book 9)
• What Happened at Midnight (Hardy Boys, Book 10)
• Hunting for Hidden Gold (The Hardy Boys, No. 5)
• While the Clock Ticked (Hardy Boys, Book 11)
• The Missing Chums (Hardy Boys, Book 4)
• Footprints Under the Window (Hardy Boys, Book 12)
• The Secret of the Old Mill (Hardy Boys, Book 3)
• The Mark on the Door (Hardy Boys #13)