Minauros

From Fiendish Codex II: Tyrants of the Nine Hells (3.5e):

A constant, oily rain pelts down on Minauros, pounding its black soil into a series of mud flats, swamps, mires, and cesspools. From these bubbling, unclean waters issue various stomach-turning stenches, each more foul than the last. The skies roil with drooping, fecund clouds ranging in color from slate gray to putrid indigo.

Muddy conditions cut ground movement rates by one-half to two-thirds, and the high winds and rain slow flight speeds by one-half. However, devils of greater or higher rank that are native to Minauros have sufficient practice operating in these conditions to retain their normal land and air speeds.

From Manual of the Planes (3e):

The third layer of the Nine Hells is a fetid swamp of mire and pollution. Acidic rain, bitter winds, and flesh-slicing hail rule the bog. The waters of the marsh are so cold in some places that they have iced over. In others, the dank water boils and steams with infernal heat.

Terrible creatures with no names swim the murky waters, and even devils fear to stray too far from their various cities. Where the layer is lowest, a constant dribble of slimy water sluices into a long fall down to the fourth layer. Many a lost traveler and petitioner has accidentally been caught up in that long, lethal fall to Phlegethos.