This effect was a slow moving ground fog done with Maya fluid effects. There was a lot of trial and error with this one. The container encompassed my entire ground plane. Then I ended up using a hidden piece of geometry as my emitter. This finally gave me the rolling effect I was going for…although it looked more like fast moving clouds than creeping fog. Transparency was increased to thin out the clouds. I tried lowering the damp to slow down the fog's movement, but it kept the fog from flowing outward instead.