if this is a basement unit, it might be damp from ground water. concrete is porous and naturally occuring water from rain, snow etc may seep through the concrete.

it might also be damp from humidity...the plastic may have trapped in moisture from humidity in the air...

or maybe something spilled and got trapped under the plastic?

I think heather has it--CONDENSATION. I don't have a fix for this-- does anyone? Melanie

Removing the mat would solve it I do think; that is if it is a condensation issue. I agree that would be a logical assessment if we knew that this was in fact a concrete slab (or basement floor)upon which the carpet was laid. I have a desk chair on wheels and I don't use a mat. No problem to me. I vacuum out the dents from the wheels.

There are mats that don't have little feet like things so that thee is a bit of airspace for hte carpet to breathe--I'm not explaining this very well, but if you go to Staples or Office Max and look at the mats, you will see what I mean. DH has a pasement office, with industrial carpeting, on a slab--one of these mats--NO MOLD OR CONDENSATE.

Good luck! Melanie

We have the same problem. I moved the mat to rearrange the furniture and noticed damp carpet. I smelled mildew in the damp area. I pulled up the carpet and nothing else besides the carpet and pad was damp or showed signs of water intrusion, either through the wall or slab. The smell however was bad. I sprayed bleach and water mix 50/50 on the pad, carpet, and slab and had a big floor fan on for hours. It dried but the room smells like the mold. What do I do now?