There is a crisis in Bayport! Soon the city will be without water! Every night water strangely disappears from the new resevoir near Skull Mountain. Frank and Joe Hardy join forces with a team 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.
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