i keep hearing about the underdark, what is it?

I am sure that there are others out there more capable of explaining this than I, but I wanted to try and help out. The underdark is a place very deep into the ground, like caves, but way deeper, and large enough for entire civilizations to thrive. This is where the famous Drow make their home, for they were once on the surface, but a war with the other races drove them deep to the underdark. There are other hideous beings and beasts that live there as well, too terrible to live on the surface, (as the authors of the books proclaim). I believe Deep Gnomes as well find home there, yet not as far down as the Drow go. Down there, there is no communion with the outside world. I hope this makes since.

(ooc)
You might also want to check out the thread's in the Baldur's Gate II board, I think this is what you're looking for.