The Stockwood Scrolls
The Underdark is a network of subterranean tunnels, caverns, seas, and rivers that spans the entire world. Lightless but far from lifeless, the Underdark is home to a dizzying array of creatures, from the civilized but unforgiving drow to carnivorous monsters that haunt the darkness.
Comprising approximately the first 3 miles of depth, the Upperdark is a realm shared by drow, duergar (gray dwarves) and other dwarves, and deep gnomes. Here the races of the World Above (as Underdark dwellers call the surface lands) intermingle with those who dwell in darkness.
The larger drow and deep gnome cities are located in this layer, 3 to 10 miles below the surface.
Below 10 miles lie the most dangerous and alien reaches of the Underdark—places that even drow seldom venture into.