In summer, bison roamed freely, almost continuously cropping the abundant green grass.
Now, that grass is not only withered and frozen, it's about to be buried.
Sixty million tons of snow now blanket this herd's territory.
Pushing through deep snow is exhausting work, and the bison are now slowly starving.
Just keeping warm saps huge amounts of energy.
Their thick coats can insulate them down to minus 20 Fahrenheit. It's now minus 40.
The only thing that will keep them alive is buried beneath three feet of snow.
And that's a problem shared with a surprising neighbour.
The food the fox seeks is also deep beneath the snow.
The survival of both creatures depends on getting through to the ground.