Instead what Innes and Perry discover is that the earth's crust in only 500 miles thick and that the inner surface is inhabited. This is the land of PELLUCIDAR, a place where dinosaurs roam through the jungles, and where saber-toothed tigers hunt the mastodon and mammoth. A tiny sun, the molten core of the Earth, hangs in the center of the heavens, shedding perpetual daylight upon Pellucidar. Because the sun never sets, because it is always now, there is no such thing as time in Pellucidar! Stranger still, because Pellucidar rests on the inner side of the Earth's crust, there is no horizon. The land curves* upwards*, as if you were standing on the inside of a gigantic bowl.

Humans dwell in Pellucidar as well, stone-age men and women who must fight to survive in this savage world. Even worse, these people have been made slaves of the Mahars, a race of intelligent but sinister reptiles who look upon humans as nothing more than beast of burden or as tasty snacks in one of their ghoulish ceremonies.

The struggle of David Innes and Abner Perry to free humanity from the Mahar tyranny is only the beginning of their adventures in Pellucidar. There are a total of seven books in this exciting series, in which Edgar Rice Burroughs takes you on journeys across savage seas infested with plesiosaurs and other hungry creatures, to mountains where pterodactyls roost, and to lands where every waking moment is a struggle to survive. Even Tarzan visits Pellucidar, taking a ride on a dirigible through "Symmes Hole" at the North Pole. So take a journey, via Iron Mole or dirigible, and discover for yourself the wonders, the terrors, and the excitement of Pellucidar…

The Chicago papers published numerous articles on Earth’s Core theories and life on Mars. 
Many have been reprinted in ERBzine.com.
Here are two examples:

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www.erbzine.com/mag14/1448.html
Into the Heart of the World After Andree
The Chicago Tribune ~ October 3, 1909

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www.erbzine.com/mag14/1446.html
“Is There a WORLD INSIDE of the WORLD?"
Chicago Daily Tribune August 3, 1913?
NOTE: Edgar Rice Burroughs commenced writing At the Earth’s Core, the first novel in the Pellucidar series, in January 1913

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There are interesting relating items in ERB’s personal library.
ERBzine’s ERB Library Project with over 1,000 titles, publishing info, bios, art, covers, etc. is featured at:
www.ERBzine.com/dan
Through the Earth by
Clement Fezandie (1865 – 1959)
is one of the volumes found there on shelf F1:
www.erbzine.com/dan/f1.html

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A search of ERBzine using our internal Google search found on the opening page or on all the Archive pages will find many more references to early proponents of the hollow earth theory.