Deeping Steppes as named by Dargain the Gnomish adventurer who happened upon this place when attempting to find lost friends in a nearby series of caves and was swept in by the unexpected surge of an underground river, perhaps due to shifting of plates or erosion of a natural barrier. The cavern was sealed and unknown until being discovered by Dargain.
This is a region of the underdark where a series of very large plateaus occupy a huge underground cavern. The cavern is situated near a very common and sometimes treacherous underdark route know as The Underway (in my version of the world) which reaches from the Dales to Silverymoon regions in the Forgotten Realms.
A society in which people believe that the hundreds of stars that dot the sky are the dreams of the sleeping. When the sun goes down, and the people of the world rest, the dreams begin to seed the sky, disappearing in the morning as the populace awakes once more. It could be just a folk tale, or it could be real...