From epubbooks.com:

David Innes is a young man who has just inherited a large mining company. An eccentric inventor, Abner Perry, convinces Innes to underwrite a project to build an ‘iron mole’, claiming it will make them both wealthy. The mechanical beast works well, actually too well. On the maiden voyage, instead of digging for a few minutes and returning, they plunge straight through the earth’s crust into the ‘inner world’ of Pellucidar. This world resembles earth but is a horizon-less, primeval tropical landscape where the sun neither sets nor rises, and is populated by ‘Sagoth’ gorilla men, wild human slaves, and the ruling hypnotic reptilian ‘Mahars’. Upon arrival at this strange world, the men are immediately captured and enslaved. But soon Perry learns to read the language of the Mahars, and discovers a secret way to turn the tables! True to Burroughs form, this non-stop fantasy thriller weaves together savage islanders, pterodactyls, telepathy, and, of course, romance.

From epubbooks.com:

In this sequel to At the Earth’s Core, David Innes vows revenge and returns to the Inner World of Pellucidar to rescue the beautiful Dian, who had been torn from his arms by trickery. However, his return trip places him far from the land of his beloved and he is forced to undertake a desperate journey thousands of miles across the fierce inner earth to reach her. David’s epic voyage takes him through the many strange lands of Pellucidar, including the pendant moon and Land of Awful Shadow. His heart pounding encounters with primeval beasts and extraordinary peoples makes Pellucidar one of the best adventure stories ever penned by Edgar Rice Burroughs.

From epubbooks.com:

Pellucidar - the hollow center of the Earth, a land of savage men and prehistoric beasts - is the scene of this exciting novel. In Pellucidar dwell the Buried People; here is the Land of Awful Sorrow; here the terrible Korsars terrorise the oceans, while dinosaurs and saber-tooth tigers terrorise the lands. This is the story of Tanar, a young chief, and the cave girl Stellara, and of their struggle for survival against a myriad dangers.

BALLANTINE BOOKS made publishing history with the simultaneous publication of the first ten volumes in the series of twenty-two adult Tarzan books by EDGAR RICE BURROUGHS.  These were followed in sequence by numbers eleven and twelve.  NOW, with the simultaneous release of the last ten volumes, the entire Tarzan series is available in BALLANTINE BOOKS editions - all complete and unabridged, all authorized:

1 TARZAN OF THE APES
2 THE RETURN OF TARZAN
3 THE BEASTS OF TARZAN
4 THE SON OF TARZAN
5 TARZAN & THE JEWELS OF OPAR
6 JUNGLE TALES OF TARZAN
7 TARZAN THE UNTAMED
8 TARZAN THE TERRIBLE
9 TARZAN & THE GOLDEN LION
10 TARZAN & THE ANT MEN
11 TARZAN, LORD OF THE JUNGLE
12 TARZAN & THE LOST EMPIRE
13 TARZAN AT THE EARTH'S CORE
14 TARZAN THE INVINCIBLE
15 TARZAN TRIUMPHANT
16 TARZAN & THE CITY OF GOLD
17 TARZAN & THE LION MAN
18 TARZAN & THE LEOPARD MEN
19 TARZAN'S QUEST
20 TARZAN & THE FORBIDDEN CITY
21 TARZAN THE MAGNIFICENT
22 TARZAN & "THE FOREIGN LEGION"

TARZAN 13  TARZAN AT THE EARTH'S CORE

Tarzan sets out with Jason Gridley to rescue David Innes, imprisoned in a Korsar dungeon in fabled Pellucidar, the world within a world, lit by its own sun deep under the Earth's crust.  Here even Tarzan's giant strength is taxed to the limit as he battles with twenty-foot flying reptiles, sabre-toothed tigers and the other frightful carnivorous ancestors of the savage beasts he is accustomed to in his own jungles.

From epubbooks.com:

Five hundred miles beneath the surface of the Earth lies another world - a world of eternal day and endless horizons, in which dinosaurs still roam and cavemen hunt and terrors forgotten in the outer world still survive. Lieutenant von Horst, member of an exploring expedition, was left behind in this lost land of Pellucidar. Back To The Stone Age is the thrilling story of his perilous adventures, along with the cavegirl he loved, in that primitive unknown world.

From epubbooks.com:

If you have ever wondered what a civilized man of the twentieth century would do if catapulted into an Old Stone Age where huge cave bears, saber-toothed tigers, monstrous carnivorous dinosaurs, mammoths, and mastodons roamed the savage terrain, you need look no further than Land of Terror, the sixth installment of Edgar Rice Burroughs’s Pellucidar series. Years ago David Innes and Abner Perry bored straight down through five hundred miles of the earth’s crust and landed in Pellucidar, the savage, primeval world that lies at the center of the earth. This is the story of their continuing adventures in the timeless land of perpetual noon and their encounters with the hideous creatures and savage men who pursue them. Although they encounter enemies at every turn, David and Abner find a few loyal friends as they embark on exhilarating adventures.

From epubbooks.com:

When David Innes and Abner Perry set out to search for mineral deposits in Perry’s newly invented Mechanical Prospectro, they never dreamed of discovering the beautiful, terrifying world of Pellucidar five hundred miles beneath their feet. Cast into a country of fierce fighting men, beautiful women, and vicious beasts, David and Abner take sharply diverging paths. David and his mate, Dian the Beautiful, set out to teach Pellucidar the ways of civilization and succeed in gathering a number of primitive kingdoms into the Empire of Pellucidar. Meanwhile, Abner turns his inventive genius to the science of aeronautics, with dire results for both David and Dian.