
As we covered in another thread, the original Ruins of Undermountain box set detailed Levels 1-3. Level 4 is the Farms Level, and has never been developed. TSR skipped over that level and detailed Levels 5 & 6 in the inappropriately named Ruins of Undermountain II: The Deep Levels box set. Unfortunately, those levels were poorly conceived, and were terribly disappointing. No other major levels were ever developed. Only a few sub-levels were done in module form or thrown into the second box set.
So there's a lot of work left to do!

The Back In Print Project is currently developing a project called Ruins of Undemountain III: The Deadly Levels. This will deal with Levels 7 & 8 of Undermountain. It will also detail two sub-levels.
We're skipping Level 4 for the time being, probably for the same reason TSR skipped over it...it's a farm!

After that, we will be developing the final of the major Undermountain releases...Ruins of Undermountain IV: Lair of the Mad Mage. This will cover Level 9 (Halaster's personal abode). It will also cover two sub-levels, including a deep sub-level run by Halaster's most powerful and (to this point in time) unnamed of his original Seven Apprentices. Level 9 is a double-sized level and extremely challenging!

At that point, we will not develop the remainder of the 14 known sub-levels that have not yet been developed. Gotta leave something for the DM to work with! Instead, we will re-visit (ie re-write from scratch) Level 5 and Level 6. These levels deal with two of Halaster's most exciting apprentices, and they were totally dreadful and uninspired in the second official box set from TSR. Poorly written, silly, tons of missed opportunities, uncreative...they were a total let-down to most Undermountain fans. We have some ideas for those levels that blow away what was slopped out by TSR.
Along with that, we are working with someone who's been detailing expansions of the undeveloped areas of first three levels that were presented in the original box set. We'll be calling that project Return to the Ruins of Undermountain, as it is indeed a return to earlier levels (Levels 1-3).
So all-in-all, a lot of work but a labor of love!

Much of Level 7 is written, and I have extensive notes done for Level 8 and Level 9, so I am actually working ahead as ideas pop into my head. I also have reams of notes for the Level 5 and Level 6 revisions.
That's what's being worked on right now. We may decide to go for broke if the momentum starts rolling and simply detail Level 4, the Farms Level. We'll see how it goes. Anyway, that's what's being planned.