In one of his most daring exploits, Jack goes down under -- literally -- to investigate the disappearance of a diver off the Great Barrier Reef of Australia. Using everything in his backpack arsenal from anti-puke pills to a morphing suit, an egg-shaped submarine to Disappearing Milk, he exposes a deadly nest of pirates raiding a shipwreck site. Jack has to thwart their schemes, evade their attempts on his life, and escape the primordial hunger of three deadly species: a Komodo dragon, a set of sharks, and the most poisonous predators in the reef, the Irukandji jellyfish. Can he save the treasure and the diver, not to mention himself? Canny and resourceful, Jack will delight readers and impress their parents.