Astronomer Edmond Halley (also known as Edmund, debate still rages over which spelling he preferred) may be best known for the comet that passes through our solar system once every seventy-five to seventy-six years (next sighting due in 2061, set a reminder in your iCal), but in 1692 he proposed an intriguing theory: that the Earth was hollow.
Halley suggested that the surface of the planet upon which we teem was 800 kilometres thick, surrounding two inner concentric shells and a molten core. He claimed that a breathable atmosphere separated these shells; that they were luminous and possibly inhabited. As ludicrous as it now seems, the theory was not definitively disproven until 1774.
The concept of worlds within has captured the imagination of writers and folklorists ever since, with prolific Western Australian poet John Kinsella now adding his name to a vaunted list of subterranean-fiction authors including Jules Verne, Thomas Pynchon, Lewis Carroll, and my childhood obsession, Edgar Rice Burroughs.