This is from Pathfinder Gazetteer (page 39 and 40 under the Mammoth Lords) and is repeated in the Pathfinder Chronicles Campaign Setting book:
"Incredible rumors suggest that the beasts that make the realm famous somehow emerged from an impossible underworld with its own false sun, a savage microcosm in which the dinosaurs never died and humans never emerged from their primitive origins. The Pathfinder Society has mounted five expeditions to this rumored inner world, but none of (sic) have ever returned from the country alive."
"Some are said to be so huge that they are themselves inverted worlds, that the denizens walk and live along the inner surface of the vault in defiance of gravity, and that their skies are lit by a glowing orb of brilliance akin to the sun itself floating at the immense cavern's center."As a fan of the D&D Hollow World setting, I really hope to see more of the inner world of Golarion developed. We'll be watching for it.