"The Hollow World is a sub-setting for Mystara. The Hollow World details the inner surface contained within the world of Mystara, similar to the real world legends of the Hollow Earth. In the Mystara setting, the Hollow World acts as a living "cultural museum" for the planet, forever preserving cultures and races that are near extinction on the surface world.
This world is lit by an eternal red sun at the center of Mystara. The existence of the Hollow World is not, in general, known to the inhabitants of the outer world. The north and south poles are actually huge, subtly curving holes that allow passage between the outer and inner world, although it is a long, hard trek through a cold, unlit, stormy and anti-magic area. The curvature of the holes is so subtle that explorers from either surface do not notice the transition until after it is already made, causing quite a shock for most.
The Hollow World was originally discovered by Ka the Preserver, an ancient Immortal who began life as a giant carnosaur, who after finding it, decided to use the inner surface of the world as a refuge and preserve for creatures that were on the verge of becoming extinct in the ever-changing outer world. Since that time, the Hollow World has become a vast refuge for cultures, and species that have become extinct on the surface of Mystara.
Within the Hollow World, characters from the surface world are severely limited by the magic used by the Immortals to preserve the stability of the various cultures. The requirements to learn magic are much higher in the Hollow world, and many spells are non-functional or unavailable. The Hollow World also adds several new player races including Beastmen, Brutemen, Kubbits, Krugel Orcs, Malpheggi Lizardmen.
The Hollow World has four major continents; Aercial, Iciria, Jomphur and Suridal. There are three major island groups; The Anathy Archipelago, The Hagvar Islands and The Wintillian Islands.
Dungeons and Dragons: Warriors of the Eternal Sun is a Sega Mega Drive game which depicts a group of adventurers who find their home transported to the Hollow World by an immortal known as the Burrower. This campaign setting is no longer officially supported."
Source: Wikipedia, "Dungeons & Dragons campaign settings", available under the CC-BY-SA License.