I thought I'd start a new thread about this, as I'd hijacked the one on HDRI and better shadows a bit too much.

Inside of a booleaned cube (think a sealed box), I have a sphere.  The mats on the sphere and cubes are all from MadMax's HDRI mats set, with no ambiance.  The sun is disabled.

In that sort of setup, there should be no light source.  I rendered the scene twice, once as described, the second time with AS's HDRI to provide light.

Upon rendering, both scenes appeared completely black (as you would suspect).  However, the HDRI version took over 50 minutes to render, which was interesting in itself.

To see what took so long, I firstly adjusted the levels of the image, and then brightened the gamma significantly.  I did so for both renders.

Come visit us at RenderGods.

Ignore the shooty dog thing.