https://forgottenrealms.fandom.com/wiki/Underdark
World AboveThe surface area of Toril. The name those who lived in the Underdark gave the surface.UpperdarkThe first 3 miles (4.8 kilometers) below the surface. It was here where the surface dwellers and those in the Underdark most often met.MiddledarkLocated 3 to 10 miles (4.8 to 16 kilometers) below the surface, this layer was where most of the Underdark cities were located.Lowerdark10 miles (16 kilometers) or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.