The Underdark was the enormous system of subterranean caves, caverns, and tunnels located beneath the surface of Faerun and, perhaps, all of Toril.
- World Above: The name that those who lived in the Underdark gave the surface area of Toril.
- Upperdark: The first 3 miles below the surface. It was here where the surface dwellers and those in the Underdark most often met.
- Middledark: Located 3 to 10 miles below the surface, this layer was where most of the Underdark cities were located.
- Lowerdark: 10 miles or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.
See https://forgottenrealms.fandom.com/wiki/Underdark for more.