The Underdark was the vast network of underground caverns and tunnels underneath the surface of Abeir-Toril. It was home to a host of evil beings driven deep into the caverns at the end of the age of demons.
World Above -The surface area of Toril. The name those who lived in the Underdark gave the surface.
Upperdark - The first three miles below the surface. It was here where the surface dwellers and those in the Underdark most often met.
Middledark - Located three to ten miles below the surface, this layer was where most of the Underdark cities were located.
Lowerdark - Ten miles or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.