For the book on this subject, see Dinotopia: The World Beneath.
Golden Caverns LO sm

The Monument Caverns.

The World Beneath was a vast network of caverns beneath the Dinotopia, the World Beneath was where the dinosaurs sought refuge long ago in order to survive the cataclysm that ended dinosaur life on the rest of the planet elsewhere. Many years later, after the techno-island-city of Posiedos was swallowed up by the sea, the survivors fled in a fleet of strutters and colonized the World Beneath, making technology-based colonies. However, after many years of civil unrest, the inhabitants abandoned their colonies, and went back to living the Dinotopian way.
Horned guardians

The Chamber of the Guardians.

Arthur Denison travelled there in Dinotopia: The World Beneath with Bix, Oriana Nascava and Lee Crabb, and found remnants of ancient sunstone-based technology such as strutters.

There were entrances to the World Beneath near Canyon City, Sauropolis and the Rainy Basin.