The Hardy Boys #7: The Secret of the Caves book by Franklin W. Dixon

There's something very strange happening in and around Bayport and the Hardy Boys are doing their best to get to the bottom of it.

It's all triggered by the disappearance of a brilliant young professor. The mystery takes the 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 its brilliant white glare, the startling secret of the caves is dramatically revealed.