Mud terrain should follow the same rules as lava regarding stuck in it and sinking in, except without the fire damage or eventual death (you can’t get submerged completely in it). Also, you can apply a towel to it repeatedly to suck out the water, which dries it up into normal floor, producing the message “Your towel gets filthy!”

It should appear starting in the early-to-mid Dungeons of Doom, so that monsters like giants can smack you into it and get you stuck.