On a windy and unseasonably warm winter day in Yellowstone National Park in Montana, spokesperson Morgan Warthin stands in the middle of a massive, empty valley.
"Yellowstone is so big," she says. "Where do you begin to look?"
She's 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.
"I didn't believe it when I was first told," says Rick Wallen, a park bison biologist.
Finding these animals won't be easy, according to Wallen, because there are thousands of bison in Yellowstone.
The only thing that sets the escaped animals apart is a tiny ear tag, which can be hard to spot from long distances.
"Bison, in the winter time, are incredibly furry animals," Warthin says. "And so the fur, being so thick, covers up those tags. So it makes it very difficult to find the tag."