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 town has a festival every year in the dead of winter. The festival of birds. The towns folk dress in feathery robes and beaked masks and dance and frolic in the belief that the changes of the seasons are controlled by the presence of birds. According to tradition, the festival tricks Spring into coming early because the birds have returned.