When I bought my house it was divided into four 1 bed flats.
As my then wife and I had split up during the purchase we ended up taking two flats each to live in until the marriage was either fixed or terminated. (We divorced in the end.)
In one of my flats there was a bathroom with a lobby and airing cupboard outside of it.
When I opened the airing cupboard door it was stuffed full of clothes and the smell of BO and 'toilets' was overpowering.
I opened the doors and windows in that flat and left it to air while living in the other flat. It took a year before it was approachable to clean out the airing cupboard.
I got some bin bags and started removing clothes. As I worked down the layer of men's clothes there were soiled pants, vomit stains, pee stains, and generally disgusting.
Then there was a layer of newspaper that dated the pile!
Under the newspaper the clothes were different but in the same state of disgusting.
Then some more, older newspaper.
Then it was women's clothes, also soiled, stained and disgusting.
Then more newspaper....
The pile went back many years, changing gender and style and the newspaper dividers were obviously when tenants changed!
I don't know how they could have lived like that!
The lobby and airing cupboard, and the bathroom, was then demolished and the lot taken to the tip!
When I started on the stinky kitchen in that flat, there was an old Belling cooker in there. I decided I could drag that across the floor and get it out and to the tip.
Half way across the floor the cooker tipped over and the oven door burst open.
Unbeknownst to me there was a chip pan in the oven, probably several years old and rancid. It fell out and poured across the floorboards!
I tried cat litter to soak it up. Then washing up liquid, then washing power, then a crowbar. The floorboards had to be ripped out to get rid of the oil and the smell, some of the joists also had to come out too. The oil had dripped into the basement and so the basement 'brick on sand' floor also had to come out.
The house doesn't smell any more!