The Underdark was the enormous system of subterranean caves, caverns, and tunnels located beneath the surface of Toril.
Upperdark The first 3 miles (4.8 kilometers) below the surface. It was here where the surface dwellers and those in the Underdark most often met.
Middledark Located 3 to 10 miles (4.8 to 16 kilometers) below the surface, this layer was where most of the Underdark cities were located.
Lowerdark 10 miles (16 kilometers) or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.