There is a crisis in Bayport! Soon the city will be without water! Every night water strangely disappears from the new reservoir near Skull Mountain. Frank and Joe Hardy join forces with thteam of skilled engineers to solve the baffling mystery. When the boys arrive at the foot of the mountain, they are met by a human skull rolling down the slope -- a chilling warning from a weird hermit. Violence, too, stalks the mountain, where a gang controlled by a powerful crime syndicate hatches a vicious plot. While trying to uncover the gang' s sinister motive, Frank and Joe aid their detective father in his search for a missing scientist. By a clever experiment, the boys discover a clue to the puzzle of the vanishing water. Suspense mounts when they explore an ancient subterranean river channel, deep inside Skull Mountain. In a thrilling climax the famous young detectives solve their own mystery as well as Mr. Hardy's case. - Flyleaf.