Journey to the interior

© BRGM im@gé/François Michel
©BRGM im@gé
© CNRS Phototheque/J-F. Ritz
©CNRS Phototheque
© ESA/DLR/FU Berlin (G. Neukum)
© ESA/DLR/FU Berlin (G. Neukum)
© CNRS Photothèque/Frank Lavigne
© CNRS Phototheque

In his cycle of six novels and 11 collections of short stories published between 1914 and 1944, Edgar Rice Burroughs travels through the fascinating realms of Pellucidar, the world that lies deep in the Earth's interior. In the fourth novel, he even has Tarzan, his most popular fictional hero, descend into this veritable "upside-down world". In these works our planet becomes a hollow sphere and the inhabitants of Pellucidar people its concave interior surface, formed by a single continent with no horizon and illuminated by a central sun.

This fictional development of human and non-human communities in the Earth's interior ceased at an age that seems to combine various chapters of our own prehistory, an age when man and pterodactyls co-habited. The Pellucidarians never came into contact with the inhabitants of the convex face until, in search of new minerals deep beneath the surface, the prospector David Innes discovered them when testing the "mechanical mole" of his inventor companion Abner Perry. A perfect digging device but which, fortunately for the reader, could not turn around.

Absurd? In all seriousness, in 1721, the Frenchman Henri Gautier, a doctor and engineer with the Roads and Bridges Authority, formulated the hypothesis that the Earth is entirely hollow - its external layer, just 5 km thick, being the result of a dynamic balance between the force of gravity and the centrifugal force resulting from our rotation. On the basis of simple findings, such as the presence of shells lying deep in the earth, he deduced the existence of an interior world with its own seas and continents and had a ready explanation for the eruptions and depressions of the Earth's crust: our external mountains were hollows in the interior world and vice versa.