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.