Good round up of hollow earth stories/exposition at greyfalcon.us
Edmund Halley was a brilliant English astronomer whose mathematical calculations pinpointed the return of the comet that bears his name. Halley was fascinated by the earth’s magnetic field. He noticed the direction of the field varied slightly over time and the only way he could account for this was there existed not one, but several, magnetic fields. Halley came to believe that the Earth was hollow and within it was a second sphere with another field. In fact, to account for all the variations in the field, Halley finally proposed that the Earth was composed of some four spheres, each nestled inside another.