In my first apartment, around 2003, my roommate and I had let the kitchen go for a week or two...and the smell was horrid. So one day we decide it needs to be done. We cleaned the entire kitchen from top to bottom, but for some reason, the horrid smell was still there. Tracked it to a cabinet above the fridge that we had never opened. Found an old bag of potatoes from the previous tenant, it was so old it had liquified. Dunno if you've ever smelled rotten potatoes, but it's bad.