UnderDark by Silence
Few areas are as forbodding and hard to get to as the UnderDark; the whole area's a nice No-Recall zone, full of thieves, murders, and general things you dont want to run into..all the more reason to explore it heavily.
The first thing to know about UnderDark is that despite it's no-recall status, it's quite large; large enough to have everything an area should have. There's also something most area's don't have - an underlying story and plot. The acutal room descriptions are average for NS - in that thier accurate, descriptive, and generally help set the mood of a dark and seedy underside of the world. But it's the stories that shine through.
There are 3 major groups in UnderDark; the Assassin Guild, the Thief Guild, and the Eye's. Theives and assassins are trying to kill each other; maybe that's why there's not terribly many of them. The beholders (giant walking eye's) apparently run the show; and thier tough as nails to boot. There's also a tribute to the imms of some other mud, i belive, on one of the streets; there's even a bit of C code carved into the walkway.
The main difficulty involved in this area, is getting to it; the directions are quite complicated, and a wrong step at any point, and a restart is necessary. Unpleasent, no? The equipment inside is fairly worthy thought, if not as good as some of the other gear to be had elsewhere. Check it out, especialy around mid-30's, to find out what a really...tense...situation feels like.