Amongst D&D players, especially those familiar with the Forgotten Realms setting, the existence of the Underdark is well-known. This world beneath the surface has even made it into many other campaigns, and into Pathfinder-related content.
I feel I must ask, since I cannot find an answer anywhere:
What is beneath the Underdark?
I'm limiting the scope of my question to the Forgotten Realms setting, since it's the canon setting as far as I'm aware.