At the Earth's Core, TANAR OF PELLUCIDAR, Edgar Rice Burroughs, (Pellucidar Series #3)

In this third Pellucidar adventure, Jason Gridley is experimenting with a new radio frequency he dubs the Gridley Wave, by which he picks up a transmission sent by scientist Abner Perry, from the interior world of Pellucidar at the Earth's core, a realm discovered by the latter and his friend David Innes many years before. There Innes and Perry have established an Empire of Pellucidar, actually a confederation of tribes, and attempted with mixed success to modernize the stone-age natives. Lately things have not gone well, and Innes is currently held captive in an enemy realm. Perry transmits a lengthy account of how this has come about, as reported by Innes’ native comrade in arms Tanar, and appeals for aid from the outer world.