Well, My Undermountain is already much larger than Biowares (and I came up with Undermountain the week after the original NWN Game came out)
But the Underdark is much further underground, and I DO plan on having a break-away tunnel that will take you into the Underdark, but it may be in the second Module (Part II)-- I don't know yet, but It WILL be much larger than Biowares for sure.. and more detailed.
I MAY have a level or two of the Underdark on the current EfU Mod (maybe level 14 or something)...
Stay tuned!