Edgar Rice Burroughs

Tanar of Pellucidar illustrated Edgar Rice Burroughs

The author's friend Jason Gridley is experimenting with a new radio frequency he dubs the Gridley Wave, via 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 modernise the stone-age natives.