1. In 1692 Edmond Halley (of comet fame) proposed that the Earth is hollow. Below the outer crust where we live, he pictured two concentric shells and a core about the size of Mercury, all floating in a luminous gas.
2. Helloooo down there: Halley even imagined that these shells might be inhabited. Jules Verne riffed on this idea in his classic Journey to the Center of the Earth.
3. Halley was right about the planet-size core, at least. At Earth's center is an iron-rich orb more than 4,000 miles wide—bigger than Mercury, actually—closer to our feet than L.A. is to New York.