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. It then follows the idea’s afterlife in the Shaver Mystery of the 1940s, Raymond Bernard’s UFO claims, Cyrus Teed’s concave Earth, a wartime German attempt to spot the British fleet by pointing cameras at the sky, and fiction from Jules Verne and Edgar Rice Burroughs to the Godzilla and Kong films.

• Why emergence myths make the underground both tomb and womb across cultures
• How Earth’s average density of 5.5 grams per cubic centimeter rules out large interior voids
• The Kola Superdeep Borehole: about 12 kilometers down out of roughly 6,400 to the core
• Cellular cosmogony and the psychological appeal of living inside the sphere
• Pellucidar, the iron mole, and why fiction gives the hollow Earth a permanent home