Edmund Halley predicted the return of his comet, and in 1692 he also argued that the Earth was hollow, built of concentric shells spinning inside one another with luminous, possibly inhabited atmospheres between them. This episode explains how flawed density figures from Isaac Newton and the real problem of wandering compass readings led a serious scientist to that model, and why he thought the aurora borealis was subterranean gas leaking out at the North Pole.

The conversation runs from ancient underworld entrances in Greece and Ireland, through John Cleves Symmes Jr.'s 1818 polar-hole theory and the lobbying that helped launch a United States exploring expedition to Antarctica, to the physics that closed the case: the 1774 Schiehallion pendulum experiment, Henry Cavendish's 1798 torsion balance, hydrostatic equilibrium, and seismic waves. ...