The Secret of the Caves (Hardy Boys, Bk. 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 their pal Chet Morton, with his new metal detector, and buddy Biff Hooper, turn up a clue that sends the young detectives on an exciting search in another direction-to the Honeycomb Caves, on the seacoast. A hot-tempered hermit who considers the caves his private domain complicates this seacoast mystery and challenges every sleuthing skill Frank and Joe possess.