Pellucidar

Pellucidar is a classic adventure and science-fantasy series by Edgar Rice Burroughs, the creator of Tarzan and the Barsoom novels. The series is set inside a hollow Earth called Pellucidar — a vast inner world lit by a miniature central sun. Because the sun never sets, time is confusing there. The land is filled with prehistoric creatures, primitive tribes, strange civilizations, and terrifying predators.