The Earth being a hollow shell, Pellucidar is the internal surface of that shell, populated by dinosaurs and primitive people. A small sun hangs at the very centre of the hollow sphere, so that it is effectively perpetually overhead wherever you are in Pellucidar. The sun never changes in brightness, and naturally never sets; so with no day/night and seasonal progression, the natives have little conception of time.
There is a Tarzan story in which Tarzan finds his way into Pellucidar.
The seven Pellucidar novels: