yeah im gonna be doing some drow characters, and drows tend to require an underdark to be about. just wanna know.
An established underdark? No, but you can definitely for sure make one up for the purposes of your characters so that they make sense. Of course, it doesn't have to be called 'The Underdark', you can call it whatever you like. ;) And some people on here do in fact, have drow characters (myself not included.)
While I do not know of any underdark, There is an Underground City, called the Clockwork City. If you would like to check that out, click of the link in my signature "Vigilant Shadows"