How to know if carpet pad is disintegrating?

I rent, so I can't just pull up a good portion of the carpet to see if my suspicions are correct. But the carpet is disgusting and smells. Cleaning it only makes it worse. I could go over and over and over the same spot and never get relatively clear water. It's just dark mud. It makes me think that pad is disintegrating under neath and the water is not getting clean because I keep getting carpet pad dust up. I want to pull this carpet out, possibly keep the hard wood floors. But if it's bad underneath, I can't afford to replace the carpet so the landlord would have to do it. I've suggested replacing the carpet before, even asked in a pleading tone, but he "never gets around to it." I've been here 6 years. Is there any legal way to force the landlord to replace the carpet? It wasn't in great shape when I moved in. It showed a definite traffic pattern, worn down, matted, but not worn through. It was sun bleached, but I'm not picky, I could deal with that. It wasn't until the first time I, as a good tenent, tried to clean it that I noticed any smell from it. Since then, the smell won't go away. It's a musty, moldly, elderly persons home smell. Sometimes it smells almost like a barn. As I said, trying to clean it again, only makes it worse. What can I do?