Aeronauts piloting an antigravity-powered craft descend through a Symmes Hole (John Cleves, Symmes) at the North Pole into the expected Hollow Earth. They contact several Lost Races, who pay them little heed. Progressing ever deeper, they learn that humanity originated in the heart of the planet, but that the more unruly elements had long ago been expelled from the inner world, and live in exile on the surface. They make their own departure through the Antarctic Symmes Hole, and crash on an island in the Pacific, where they write down their experiences, depositing the resulting manuscript in...