A campaign setting for Dungeons & Dragons RPG (the original, not AD&D.)
The planet of the world of Mystara (the campaign world found in D&D, as described in the zillions of supplements) underwent a cataclysm. Various Immortals thought it was a pretty good idea to carve the world hollow, to create a sanctuary of sorts for Mystaran races doomed to extinction.
So, the planet is, in fact, hollow: there are large openings in the polar areas, shrouded in mist, and the openings are so smooth that people can travel into the Hollow World easily and without even noticing. The crust of the planet has magical gravity; In the centerpoint of the now empty sphere, there's an "artificial" sun.
The hollow world is an interesting place. The sun, of course, never sets, has a bit of reddish hue, and the horizon (of course) curves up. Some of the magic doesn't work, or works differently.
Various races live in the inside world; Some are modelled after the ancient Earth races, like the races modelled after iron age hunters, Mesoamerican Indians (complete with Mayan ball games, on which the players can participate in - voluntarily or more often not!), and ancient Greeks. There are also some are strange modifications of existing fantasy races (like the outre faddy fashion-following elves, the fatalistic pacifistic philosopher elves, and such)...
A very interesting world for the old and obsolete game, in TSR's scale =)