I'm trying to make a low lying fog, and the box I used as a boundary on the smoke rendered as a solid object, blocking everything inside it. How can I fix this?
As an alternative to smoke you can use volume scattering.
In an example like this:
Add a mesh that uses a volume scatter node as the volume.
Using a gradient and a color ramp you can control the height and density of the fog.
Using additional textures to affect the transparency of the volume scattering you can get a less homogeneous mist.
To move the fog animate the mapping used on the textures.