a putrid stink
I once took a train trip over the mountainous spine of New Zealand's south island from Christchurch to the west coast. At a certain point high in a mountain pass I was standing on the platform at the rear of our car when a terrible smell came upon us. As the train progressed this became a powerful reek of death and decay. For maybe three hundred metres the smell grew to an unbelievable stench, a choking thing that evoked the gag reflex in all of us outdoors. I fled into the train car, desperate for fresher air.
Gasping for breath inside the car I saw, wedged between a tree and a barbed-wire fence, a dead cow. A lone dead cow had created that smell in a dry, cool environment.
The thin mountain air thick as smoke, a living thing that slithered down your throat and left a greasy feeling with every breath. It had turned the crisp clean air of a mountain pass into a fetid miasma. A wretched, vomit-inducing thing. I had no idea that anything could be so awful.
As we passed, so too did the smell. But not soon, and certainly not soon enough.
To this day I can remember that feeling of the body wanting not to breath. I doubt I've conveyed how powerful it was.