It's like a nasty loaf of bread that got forgotten for months and molded over, then rolled in old shit.  Always smelled like peppermint oil to me, but that might just be my mask. Seriously, your description seems about right to me. Crusty old shit with a bunch of mold is about the right way to describe the smell. There were times I ended up putting something, anything to cover the smell, on the inside of my nostrils when I couldn't wear a mask. They just smell so bad.