On a windy and unseasonably warm winter day in Yellowstone National Park in Montana, spokeswoman Morgan Warthin stands in the middle of a massive, empty valley.
"Yellowstone is so big," she says. "Where do you begin to look?"
She is searching for any of the 52 bison that were set free from two holding pens in mid-January.
Authorities say the bison escaped after somebody used bolt cutters to open up a fence. They soon scattered across an area larger than Delaware, and officials have launched a criminal investigation to find out what happened.