The Articulate Plane
The Stonesea Underdark
A land as deep as it is wide running in the mountains between Kauthia and Gryttrsmid. Much of it was patiently carved by the dwarves, but it is now host to dark terrors of every description. It is run primarily by the duergar and their mind flayer overlords. Slave trading is a primary industry there.
A dangerous but lucrative smuggling industry moves goods through the Stonesea. Anyone capable of guiding a caravan through it safely can make some good money.