From http://strangemaps.wordpress.com/

If the Earth is hollow, where does all that magma spewing out of all those volcanoes come from? Somebody must have a half-convincing answer to that question, presumably that handful of people who still believe the Earth is an empty shell. The idea seems quite ludicrous now, but in pre-scientific times, it at least appeared to make sense: if Heaven was a place in the skies above, where else would Hell be than somewhere deep below our feet?

Harder to understand is why the idea survived several centuries of scientific progress, including the powerful notion of nature’s horror vacui. In a 1692 scientific paper, Edmund Halley – yes, he of comet fame – put forth the idea that Earth consists of a shell about 800 km thick, and of two inner concentric shells and an innermost core with about the same diameter as the planet Mars.

Halley did have scientific grounds for his rather bizarre thought-construct. It tried to explain why compass readings could be so anomalous: each of the inner spheres had their own magnetic poles and rotated at differing speeds. To compound his error, Halley proposed that the inner spheres might be inhabited and that the inner atmosphere was made up of luminous gases that, when escaping outward, cause the Aurora borealis.

Later theorists came up with variations to Halley’s model. In the seventeenth century, Leonhard Euler proposed a single-shell hollow Earth with a small sun (1.000 km across) at the centre, providing light and warmth for an inner-Earth civilisation. Others proposed two inner suns, and even named them: Pluto and Proserpine. ... more here.

I never knew Edmund Halley wrote a paper on it. Strange indeed.

Steve G.