Sages and scholars subdivide the Underdark into three distinct layers: the Upperdark, the Middledark, and the Lowerdark.
Upperdark
The Upperdark refers to the portion of the Underdark closest to the surface. It extends approximately three miles underground, and it is in this area that surface dwellers and those who live in the Underdark most commonly meet. Underdark civilizations that do not have sunlight sensitivity, like Bugbears and Goblins, are more likely to make their homes in this part of the Underdark.
Middledark
The Middledark refers to a region of the Underdark that begins about three miles below the surface and extends to about ten miles below the surface. Intelligent hominids with sunlight sensitivity like the Drow and Duergars may have outposts and even small settlements in the Upperdark, but most of their towns and cities will be in the Middledark.
Lowerdark
The Lowerdark realms of the Underdark begin about ten miles beneath the surface and extend to unknown depths. It’s home to such fearsome monstrosities that even residents of the Middledark rarely travel that deep. Some scholars believe that there may be portals to the Abyss, the Nine Hells, Hades, and other realms of the Lower Outer Planes located in the Lowerdark. This, however, is disputed by some sages who argue that armies of devils and demons would have marched through them to conquer Adnati if they existed.