I have always resented the short shrift Wizards of the Coast gave in its "Undermountain" gave to level 4 the sewer level. Iconic "Old school" D&D always involved battling a diverse clean-up crew of Otyughs, ropers, gibbering mouthers and black puddings the size of horse carts. Here is the "official" map of the fourth level of Undermountain:
The twisty passages and river are alright. However notice the scale one square equals 10' On my standard 5 to the inch quadrille this is barely 8.5" by 11". Comparing it to the double quad fold 34" by 22" of level 3 the Skullport/Sargoth level and considering it supposed to drain and feed all of Undermountain, this map just pathetic. Fortunately it appears I am not the only one with this option. Here is a redrawn home brew fourth level of a size to do it justice (from slade88green's image shack account).
and half of it redrawn in the box set trade dress (courtesy of Doirche [who died too young] on the cartographer's guild forum). I had to use the way back machine to resurrect this one.
Undermountain Back in Print project. Although most of this site seems frozen amber, it is a least still there. Halaster is promising progress in 2016.
Now all fourth level needs is some farms, a temple of the slime god, black puddings the size of horse carts and so om.