Hollow World is a campaign setting boxed set describing a vast new territory, the hollow inside of the Known World, mapped on four large sheets. The 128-page "Dungeon Master's Sourcebook" described how in the Hollow World, prehistoric animals abound, as do strange races, such as native tribes, strange elves, and the remnants of the ancient Nithians; magic behaves somewhat differently in the Hollow World, and the people there worship different Immortals. The 64-page "Player's Book" has rules for creating Hollow World characters, plus new spells. The 32-page "Adventure Book" lists new monsters (mainly prehistoric ones), and includes an introductory scenario. The Hollow World set is also suitable for use with AD&D 2nd Edition rules.