- You may be looking for the Underland.
The Deep Realm was the first cavern in the Shallow Lands which was above Underland. It was an enormous cavern, that contained tree-like fungi that spread out into a forest. It was also the central cavern, in which creatures such as birds, bats, lizards and small dragons slept in (Note: Many were said to have looked like the bat or dragon sort, therefore they may not have been these actual ceatures, but relatives).
According to the Warden of the gnomes, they were beasts that fell into the underworld and fell asleep, and would not awaken until the end of the world (the whole world, not just Underland). Jill Pole, Eustace Scrubb and Puddleglum were escorted through this vast cavern by the hypnotised gnomes, led by the Warden after they fell into the Underworld from the Ruined City of the Giants.
In the year 2555, when the world had ended, Father Time awoke so they could destroy the trees and shrubs of the whole world, before they died and rotted into bones. Their souls then carried on elsewhere with Father Time's soul.