Put some dirt and water in a sealed plastic container and leave it on a window shelf for a week and then give it smell, it's probably something like that.

smell of rotten flesh and old pond

Bad enough to gag a maggot...

Grandmas....ahhh, nevermind

You use the toilet in the morning and forget to flush it. it sits there throughout the whole day unattended.
Then you come home and are in urgent need of the toilet. You lift the lid and the smell hits you like a surprise boxer popping out of a spring box.

That smell.

it would smell like an outhouse that was left to rot for years after a concert or the basement of a paper mill (it's been years since i worked at one and my sinuses still have not recovered from it lol)

these are all oddly accurate (AND VERY SPECIFIC) responses. i love it