The divers travel inland to Peru's Lake Titicaca, testing experimental techniques for diving at high altitude for the first time. Deep in the lake they discover a unique species of frog whose lungs have atrophied like fish, they die when removed from the water. Cousteau's divers investigate the lake's legend of Inca treasure. The treasure proves to be illusory, but ancient pottery and a unique species of aquatic frog are found.
