"We're trapped a mile below the ocean's surface," Tom Swift announces to his companions as calmly as possible. His father and Bud Barclay exchange fearful glances in the air-bubble elevator stopped in its undersea descent by a jammed cable.

       This close call is only one of the many hazards which beset Tom on his most challenging deep-sea venture––a trip to the ancient city of gold on the ocean bottom.

       Here, with his pal Bud Barclay and other members of the Swift expedition, Tom tests his two new inventions. With the de-organic-izer, which employs revolutionary scientific principles, crusts of centuries-old sea growth are removed from the city's buildings and the original gold luster restored. With his other astounding invention––the spectromarine selector––Tom searches for a valuable new rare metal. He must succeed, if his father is to fill an important government rocket contract.

       A terrifying sea monster, an abandoned submarine containing human skeletons, a near-fatal leak in the hydrodome over the scene of operations, and the unexpected appearance of an enemy fail to daunt Tom in his mammoth undertakings.

       How the young scientist-inventor achieves his goals is told dramatically in this thrill-packed story of undersea adventure.