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 most other sages who argue that armies of devils and demons would have marched through them to conquer Adnati if they existed.