TL:DR - 6 square feet of old cat piss in carpet neutralized, can have guests over that will also come back! Gather around, children, as I tell you a tale of a woman who was lifted out of a pit of madness and despair... I took over the lease for a friend-of-friend's(FF) apartment after they moved out of state for work. This FF had two cats and one nasty litterbox in the tiny bathroom, and while I did check the place out while they still lived in the unit, I thought the bathroom was just the issue. I admit, I screwed up royally when I told management that "no, I don't want cleaning to happen, I'll handle it," as I firstly thought I was taking the unit as-is in the transfer so to keep the rent low, and secondly didn't want to hurt this FF's deposit as they did me a solid favor in transferring their lease. Besides, I told the FF to not worry about leaving items behind or a mess, I'd happily take care of it, and also knew even if the professional cleaning happened I'd STILL do my own deep clean. I really wish I let management just replace the carpets before I ignored his warnings/questionings and signed the waver. While the bathroom was the deathzone with litter literally cemented to the 1in hex tile floor, upon opening the front door for the first time since the FF moved out, it was a boot made of cat piss to the face. I guess the day we visited they had every window and door open and fans going cause it was not that bad outside of the bathroom. Rented a caret cleaner the following day and got the large bottle of pet specific formula. Did a double pass over all carpets, scrubbed everything down, mopped the hell out of that bathroom and walls with vinegar, baking soda and solvents, left fans going and windows open for 36 hours. Returned with the first car load of stuff to be harshly booted to the face once again, all that cleaning did NOTHING! Not a dent. Fortunately I was straddling both leases for the month and didn't need to sleep in the new one yet, so I decided to do research before going to the management on hands and knees pleading to help me. Read about a 4 step process to neutralize cat stains which involved baking soda, followed by vinegar, then after that was cleaned up you did a short treatment of peroxide mixed with dish soap, then within 24 hours of that drying was the most important step: a cat urine neutralizer. Looked up what that really was, turns out it's enzymes and bacteria that literally make dinner out of the crystals in cat pee that get sticky and smell forever. Think of it like a smelly bowl of sugar and ants. You can wet sugar, it will dissolve, maybe you can soak up some of the sugar water, but when its dry you have that sticky mess crystalizing once again. But the neutralizer works like a bunch of sugar loving ants which get in there, eat up everything and then leave! In the world of pet neutralizers, Nature's Miracle was what everyone swore by. So locally I picked up the only cat specific product I could find by them (a can of carpet foam) and decided to try the 4 step method in the bathroom as it remained the worst offender, using the foam even on the walls cause why the hell not. After some time of doing the steps and waiting in between I could not longer tell if it was actually working or my nose went into a coma, because it seemed as if the bathroom was now the best smelling room and I puttered about. I decided to call it a day and closed up EVERYTHING. I closed all the windows I had left open for 96 hours now, closed every door inside the unit from rooms to cabinets, turned off all the fans, and just left the place for 24 hours. I had reasoning behind the closing everything off. I wanted to make sure that when I came back I could identify which rooms had the offense. I didn't want any "oh, maybe it's better cause I had the windows open... Oh, maybe this room smells cause of the other room..." The next day my fiance came to help me with the next round of item moving, and I could tell he was skeptical of my saying "I think the bathroom was smelling better..." on our way there, and that's fine, I was too. While I waited with the car in the ally and started unloading it, he went in with the first box of stuff and the keys. He took a long time to come back, but when he did: "I don't know what sorcery was in that can, but the bathroom IS the best smelling room in the apartment. AND YOU HAD ALL THE WINDOWS AND DOORS CLOSED?!!" Bingo. That was the exact reaction I was aiming for when I closed everything off. We continued to keep the doors closed after that so to pinpoint what was now the worst locations and that night I researched more and order this product specifically. Once it came in, I started the nose to carpet hunt, doing a 3x3 foot section at a time before my nose would shut down, and sure enough I found two large areas of carpet with consolations of cat pee, both being abut 1x3 feet in area outside the bedroom door and from there going into the living room. Instructions said to just soak the carpet so it got to the underflooring and leave it. I went one step further and even walked on the carpet to sponge it around and deep in there, put markers around the edges of what I treated so no one else would end up with wet socks and it took about 48 hours to properly dry up. And guess what? 3 years of old cat piss. Gone. Replaced by the scent of the cleaner, which not entirely pleasant, is far more acceptable than piss. I only used about half the bottle and it saved me having to pay to have carpets ripped out. I should be completely transparent here. While yes, it did make that big of a difference and is worth every penny and then some, I would buy it again in a heart beat, I either have yet to find all the spots, or on some of the more particularly hot days we had there was a flare up of the cat piss from deep in the carpet I didn't reach, but seriously, this stuff was a miracle. I have tried sniffing the same spots where the pee was to still smell only old carpet. It's been about a month now since I treated, I still smell the cleaner in part upon entering the apartment after being gone a day, sometimes the smell of cats wafts by, but that's okay compared to what I started with, and it goes away with open windows or air freshener.