Volo's Student's Guide to
The Hollow World
Deep inside Mystara is the Hollow World. Numerous cultures, long extinct on the surface (like the Chainmail rules) still exist here, preserved by the timeless nature of the world's interior. It was discovered by a dinosaur (no seriously), who achieved divine ascension (no... seriously!)
Okay, don't believe me! Go there yourselves! You'll see. Once you've been eaten by a prehistoric Intermediate God, you'll learn to take these travel guides seriously.
Sheesh. You'd figure after Spelljammer, you'd be used to weird things by now.