MANY ANCIENT CULTURES BELIEVED THAT THERE WAS an underworld of some sort beneath us, whether it was the Hell of the Christian tradition (Sheol to the Hebrews), the underworld caverns of the god Hades of the ancient Greeks, Cruachan of the Celts of Ireland, Patala of the Hindu tradition, Svartalfaheimr of the Nordic mythology, or Shamballa in the Tibetan Buddhist legend. In some cases the underworld was a place where people went after death, while in other cultures the ancestors of living people emerged from the underworld.

Some of these legends were apparently inspired by the mystery of large caves that could not be fully explored, or descended into darkness beyond the spelunking ability of the curious. Sometimes these were mysterious mountain caves, while others appear to be based on the drained conduits from volcanoes called lava tubes. Caves have always held a fascination and a terrifying mystery for many cultures, especially when their early explorers found the bones of dead people or animals in them. Indeed, the mystery of caves is a common theme in literature, from Plato's cave allegory, to Dante's Inferno ("All hope abandon, ye who enter here"), to Tom Sawyer, Becky Thatcher, and Injun Joe getting lost in a cave in the climax of Mark Twain's novel Tom Sawyer.

From these legends, it is not surprising that most people, not only in fiction but even in the early scientific literature, imagined that the mysterious caves of the earth led to a great hollow world beneath them. The famous astronomer Edmond Halley (best known for correctly figuring out the path of the comet now named for him) also had some questionable ideas as well. He thought the earth was a hollow shell about 800 km (500 miles) thick, with two additional inner shells surrounding a solid metal core (Fig. 1 A). From this geometry, he imagined that these shells had their own magnetic fields, and that the aurora borealis was formed by gases escaping from these hollows inside the earth. Science fiction and fantasy writers L. Sprague de Camp and Willy Ley claimed that the great mathematician Leonhard Euler thought that the hollow earth had its own sun inside. Sir John Leslie imagined there were two central suns inside the earth. This last idea inspired Jules Verne when he wrote one of the first works of science fiction, Journey to the Center of the Earth, in 1864.

Early Theories of a Hollow Earth

In the 19th century, there were many more theories about what the inside of the earth was like. One of the most famous and influential was that of the War of 1812 veteran John Cleves Symmes, Jr., who postulated that the earth is a hollow shell about 1300 km (810 miles) thick, with an opening at each pole that was about 2300 km (1400 miles) across. Inside this were four additional inner shells, also with openings at each pole (Fig. 1 B). Symmes wrote: "I declare that the Earth is hollow and habitable within;...