its probably something to do with the way your shaders are set up. Try taking a ground shader from another map and applying it to the floor of your map with your bitmaps plugged. Post results
Oh, I had this before, waaaaay waaaay back... crap, what was the problem there...
It must be either the bump map, the specular, or the alpha (if your textures have an alpha). Crud, I remember having this problem, and I have no idea what I did to fix it.
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