Summary:

This is the 13th book in the Tarzan Series and the 4th book in the Pellucidar Series. In response to a radio plea from Abner Perry, a scientist who with his friend David Innes has discovered the interior world of Pellucidar at the Earth's core, Jason Gridley launches an expedition to rescue Innes from the Korsars (corsairs), the scourge of the internal seas. He enlists Tarzan, and a fabulous airship is constructed to penetrate Pellucidar via the natural polar opening connecting the outer and inner worlds.