These giant potholes were left behind by blocks of buried ice that melted, leaving hollows that later filled with water.

 

For thousands of years since then, they've been vital pit stops for migrating birds, the millions of cranes, geese and ducks that follow ancient routes across the plains.

 

These rolling hills are made of debris dumped by the retreating ice. lt also left rich grasslands able to support hundreds of millions of bison, vast herds drifting with the seasons, always searching for fresh grazing.

 

Pronghorn antelopes are unique to North America and among the toughest creatures on the plains. Surviving year round in the open, they must cope with freezing winter temperatures and scorching summer heat.

 

Every spring these open spaces echo to the sounds of courting sage grouse, as males compete to win a mate. Migrating Sandhill cranes strut and dance for their partners before heading further north towards their nesting grounds.

 

Small birds breed here, after an epic journey from their winter home. These cliff swallows have flown thousands of miles from South America.