Add a new world datablock and enable environment lighting.
Render the image.
See the pixel artifacts in the attached image. The .blend file has been attached as well.
Disabling anti-aliasing makes most of the artifacts disappear, but depending on the camera angle and the image size, bad pixels still occur occasionally. Full sample anti-aliasing does not fix the problem.
Anyway, my computer is in the middle of crunching a bunch of data so I can’t really check out your .blend right now. Are the normals pointing the right direction?
Edit:
I just opened it up and I get the same thing. If I apply the solidfy modifier then check the normals, they are correct. Seems like a bug to me.
I tried a couple of different things, in the world panel, under gather, switch to raytrace instead of approximate and the problem doesn’t happen, but the render is grainy. Jumping the samples up to 30 fixes the grainy noise, but increases render times like crazy. When you hover your mouse over approximate, the tool tip reads ‘inaccurate, but faster and without noise’. So I don’t know if it’s a bug or just the result of using approximate instead of raytrace.
I did away with the environment lighting and positioned a lamp above the hole and turned on ambient occlusion. Same effect, the problem goes away when switching to raytrace…
I dunno, sorry, I don’t know that much about lighting…