...The secret of the caves

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. The case that their famous detective father is working on -- suspected sabotage at the new radar station near the Hardys' hometown of Bayport -- further complicates the mystery, which grows more baffling with every chapter. After many dangerous encounters, including a nearly disastrous patrol on the stormy ocean, 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. - Flyleaf.
Frank and Joe solve a double mystery by locating a college professor who has been kidnapped by foreigners involved in sabotaging a government radar installation.