40d:Mud

Indoor mud is created by filling a stone floor tile with water, then letting the water evaporate. Outdoor mud is created by covering any soil tile with water. Creating mud is essential to providing irrigation for indoor crops. The most common sources for mud are the results of irrigation, farming, and draining water sources.

Mud will spread as your dwarves and animals walk through it and adjoining tiles. It can quickly spread from a farming area throughout your entire fortress, turning floor tiles brown. Floor Grates over a channel or bridges seem to stop the spread.^{v0.27.169.33d}

Outdoor mud will completely dry over the winter if it lies undisturbed for the entire time. Barring your dwarves (and therefore any pets) from going outside is the only way to allow the outdoor mud to completely dry. If mud exists within your fort, it will dry during this time. Indoor mud can also be cleaned by dwarves with this profession enabled.

Example of Stopping Mud Spread

WWWWWWWWWWW
W=====WWWWW
W=====WWWWW
W=====Wó.óW
W=====WW.WW
W=====+#...
W+WWW+WWWWW
W1WWW7WWWWW

Code:
W - unmined [[wall]]
= - farm plot
+ - doors (water inlet/outlet, farmer entance)
# - floor grate
7 - water source
1 - water drain
ó - levers connected to doors
. - clean(!!) ground

Remember that a channel must be built in the tile before the grate is installed. I guess mud can originate from a number of sources besides your own farming, but with enough grates, the fancier parts of the fortress should be safe.

In adventurer mode mud objects which you can pick up from muddy tiles are edible. ^{v0.27.169.33c}