Pullucidar
Pellucidar, as every schoolboy knows, is the creation of Edgar Rice Burroughs, a hidden world existing within our own, five hundred miles down at the Earth's core. David Innes and Abner Perry penetrate the Earth's crust in a mechanical mole and travel to the center of the earth, emerging in a prehistoric place. There they fight for existence against stone-age dangers and try to bring civilization to this world. Their adventures are told in the seven books of the series; AT THE EARTH'S CORE, PELLUCIDAR, TANAR OF PELLUCIDAR, TARZAN AT THE EARTH'S CORE, BACK TO THE STONE AGE, LAND OF TERROR, and SAVAGE PELLUCIDAR.
Pellucidar has an inside diameter of 7000 miles with a combined ocean area of 41,370,000 and land area of 124,110,000 square miles. This is more land surface than the outer world surrounding it! In the center, always above, hangs Pellucidar's sun, creating eternal afternoon. Without night to mark its passage, the people have developed no concept of time. The Dead World, Pellucidar's moon, revolves with the Earth, always a mile above the same spot. In perpetual darkness, this area is known as "The Land of Awful Shadow". Pellucidar has no horizon since her surface curves upward in all directions until lost in the haze of the distance. A polar opening exists in the far north connecting Pellucidar to the outer crust. It is believed that the ancestors of the colorful pirates, the Korsars, came through this opening from our world.
The atmosphere near the surface is slightly denser than ours but also shallower so it’s very cold on mountain heights. Prevailing winds normally blow north to south for half of our year and then the reverse for the other half. Clouds are very rare. It’s always springtime in Pellucidar.
Pellucidar is three-fourths land and very sparsely populated. Although one could travel for many sleeps without encountering another person, the land teems with plant and animal life. A veritable melting pot where animals of nearly all the geological periods of the outer crust exist simultaneously. Beasts such as the wooly mammoth, the pteradactyl, the sabertooth tiger, and the mighty Tyrannosaurus Rex give the human races of the inner world a slim chance of survival.