We have a rubber mat that levels out 1 piece of furniture in our basement. We put the litter box on there, and the cats ended up urinating on it when we went away and our friend did not clean the box as often as they liked... We were unable to remove the mat due to the furniture, and no matter how much we cleaned it, every time it got wet it smelled horribly. It also had a general urine smell that was unpleasant the rest of the time.

After trying everything, I sifted (heavily) baking soda all over it and let it sit. It pulled all the urine and odor out of the carpet after 2 applications. It was difficult to vacuum however, due to the bottom-most layer becoming very paste like. I did find that the small tip normally used for cleaning corners and crevices did the job with a little effort.