The young scientist-inventor embarks on one of the most challenging missions of his adventure-packed career. The mission: recover a valuable cache left on Earth thousands of years ago by colonists from another planet. Can Tom locate it before his deadly foe the Kranjovians seize the capsule?

Clues to the cache lead him to a location beneath the ocean floor -- near strange Easter Island, with its eerie ruins. Further search is impossible until Tom can invent and build a manned, burrowing mole-mobile -- the Geotron.

Tom and his close pal Bud Barclay race against time in an equally challenging project: to build a unique aquarium and stock it with rare species of deep-sea life. If they fail to meet the deadline, a donor's bequest goes to the despotic Kranjovian government.

Their desperate contest with the Kranjovians propels Tom and Bud into a series of hair-raising perils. Tom's weird ordeal as a "bird man" of Rano Kao; the boy's narrow escape from being buried alive in a cave; their life-or-death undersea duel for the priceless cache -- all go to make up this electrifying, suspense-filled adventure.