The Old Faithful area in Yellowstone National Park displays continual geothermal activity.  The photograph above is not of ground fog but rather steam from the superheated water that works its way to the surface in the form of hot springs and steaming fumaroles. Below, more steam with a backdrop of a rainbow which appeared after a fast moving thunderstorm moved out of the area.

The water that boils to the surface creates run off, leaving minerals and thermophilic microorganisms that form interesting patterns and colors.