Pellucidar

In Burroughs' concept, the Earth is a hollow shell with Pellucidar as the internal surface of its shell. Pellucidar is accessible to the surface world via a polar tunnel, allowing passage between both the inner and outer worlds through which a rigid airship visits in the fourth book of the series. Although the inner surface of the Earth has a smaller total area than the outer, Pellucidar actually has a greater land area, as its continents mirror the surface world's oceans and its oceans mirror the surface world's continents.

The peculiarities of Pellucidar's geography are caused by the concave curvature of its surface. There is no horizon; the further distant an object is, the higher it appears to be, until it is finally lost in a void of atmospheric haze.

Pellucidar is lit by a miniature sun suspended at the center of the hollow sphere, so it is perpetually overhead and gives a sensation of eternal noon wherever one is in Pellucidar. The sole exception is a region directly under a tiny geostationary moon of the internal sun. As a result, this region is under a perpetual eclipse and is known as the "Land of Awful Shadow". The moon has its own plant life and (presumably) animal life, and either has its own atmosphere or is simply suspended within Pellucidar's. The miniature sun does not change in brightness and never sets. As a result, with no night or seasonal progression, the natives have little concept of time. The events of the series suggest that time is elastic, passing at different rates in different areas of Pellucidar and varying even in single locations. Also, several characters living in Pellucidar who are originally from the outer world appear to age slowly and exhibit considerable longevity. This is known through their interactions with visitors from the outer world, where time remains fixed.