I used to do plumbing when I lived in New York, and once had the owner of an apartment building unlocking apt doors with his master key so I could isolate the source of a constantly hissing water pipe, even when no one was home. (6 apartments) ALL the tenants said they had no problem, so he decided to look for it. We found it. It was in a bathroom ceiling, a badly leaking hot water shut-off valve, running down into the bathtub. The ceiling had collapsed.
The entire apartment was 3 to 4 feet deep in garbage, with paths through it. The stench of rotting garbage was almost as bad as the stench created by the cat. A smell of ammonia that would cut your breath short.
The owner hired some laborers to empty the place, and it took 3 days to get it cleaned, and 2 dump truck loads.
Do a search on the Colyer brothers from New York City. That's another classic story. With photos. And they were millionaires.