The Secret of Skull Mountain

Bayport faces a water crisis as a new reservoir mysteriously empties near Skull Mountain. Frank and Joe Hardy team up with engineers to investigate, only to be met by a rolling human skull and a chilling warning from a hermit. As they uncover a crime syndicate's plot and aid their detective father in a missing scientist case, the boys must explore an ancient subterranean river to solve the baffling mystery of the vanishing water.