The Hollow World is the inner surface contained within the world of Mystara (similar to the real world legends of the hollow earth). In Mystara, 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.
The Hollow World was originally discovered by Ka the Preserver, an ancient Immortal who began life as a giant Carnosauria, 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 Lizardfolk.