A brilliant professor vanishes, leaving behind only a cryptic message scrawled on an exam paper. Just when the Hardy brothers, Frank and Joe, are about to hit a dead end, a new clue surfaces, pointing them toward the mysterious Honeycomb Cliffs. Danger and intrigue await them as they tunnel into cavernous depths, determined to unearth the secrets hidden within. Join the boys for an exciting but treacherous adventure in book seven of the Hardy Boys mystery series.