Why U.S. Border Patrol only catches 54% of the people crossing illegally from Mexico

José Hernandez is driving down the dirt road on the California side of the U.S.-Mexican border, with his head hanging out the window, staring at the road ahead and to the side of the truck. When you've spent more than 15 years hunting humans, you get pretty good at it. 

"You're looking for overturned rocks, twigs that are broken, leaves that have dew missing, things of that nature," he says.

Hernandez has been a U.S. Border Patrol agent in Arizona and California since 2001. It takes only about eight minutes to spot something.

"You can see footprints starting on the side over there and walking down the middle of the road."

Border agents see the same types of footprints so often they've given them nicknames. The person who crossed this road left an imprint bearing the waffle pattern of a peanut shell. So this footprint, Hernandez says, is a "Mr. Peanut."