*shakes head* It's not the stuff they used to clean, it's the _reason_ they need to clean.

The previous tenants apparently had poorly behaved cats (or possibly a poorly leak proof cat box), and the carpet (and apparently the pad under the carpet) where the box was is quite stinky. They apparently replaced that, and are going to clean again, but apparently it's difficult to clean the pad from such things.

Not sure, but I also suspect that the cats urinated in locations not near the cat box. And yes, the carpet could be replaced, but that would make the place no longer in my price range.