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.