Pellucidar is a fictional Hollow Earth milieu invented by Tarzan creator Edgar Rice Burroughs for a series of action adventure stories. In a notable crossover event between Burroughs' series, there is a Tarzan story in which the Ape Man travels into Pellucidar. The stories initially involve the adventures of mining heir David Innes and his inventor friend Abner Perry after they use an "iron mole" to burrow 500 miles into the Earth's crust. Later protagonists include indigenous cave man Tanar and additional visitors from the surface world, notably Tarzan, Jason Gridley, and Frederich Wilhelm Eric von Mendeldorf und von Horst. Geography: In Burroughs' concept, the Earth is a hollow shell with Pellucidar as the internal surface of that shell. Pellucidar is accessible to the surface world via a polar opening allowing passage between 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 an absolute smaller 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 continents. A peculiarity of Pellucidar's geography is that due to the concave curvature of its surface there is no horizon; the further distant something is, the higher it appears to be, until it is finally lost in the atmospheric haze. Pellucidar is lit by a miniature sun suspended at the center of the hollow sphere, so it is perpetually overhead wherever one is in Pellucidar. The sole exception is the region directly under a tiny geostationary moon of the internal sun; that region as a result is under a perpetual eclipse and is known as the Land of Awful Shadow. This moon has its own plant life and (presumably) animal life, and hence either has its own atmosphere or shares that of Pellucidar. The miniature sun never changes in brightness, and never sets; so 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 locales. Culture: Pellucidar is populated by primitive people and prehistoric creatures, notably dinosaurs. The region in which Innes and Perry initially find themselves is ruled by the cities of the Mahars, intelligent flying reptiles resembling Rhamphorhynchus with dangerous psychic powers, who keep the local tribelets of Stone Age human beings in subjugation. Innes and Perry eventually unite the tribes to overthrow the Mahars' domain and establish a human "Empire of Pellucidar" in its place. While the Mahars are the dominant species in the Pellucidar novels, they seem confined to their handful of cities. Before their overthrow they use the Sagoths (a race of gorilla-men who speak the same language as Tarzan's apes) to enforce their rule over the human tribes within the area which they rule. Races: Pellucidar also harbors enclaves of various nonhuman or semi-human races. There are: The Ape Men - A race of ape-like black creatures with prehensile tail and are arboreal. The Azarians - A race of primitive man-eating giants. The Brute-Men - The Brute-Men are peaceful gorilla-like farmers. They are sometimes called "Gorilla-Sheep" for the sheep-like appearance of their faces. The Coripies - A subterranean race, also known as the Buried People, the Coripies are a race of short eyeless carrion-eaters. The Ganaks - A race of horned bison men. The Gorbuses - A subterranean race of cannibalistic albinos who are apparently resurrected surface-world murderers. The Horibs - A race of ferocious dinosaur-riding reptile men. The Mahars - The master race of Pellucidar who resemble Rhamphorhynchus. The Sabertooth Men - A race of ape-like black creatures with prehensile tails. They are cannibals with dagger-like tusks. The Sagoths - The gorilla-like servants of the Mahars."