End of the fence

The United States has built a fence across much of the frontier between Arizona and Mexico, designed to make it much harder for people to cross the border there illegally. But when the frontier becomes too steep, the fence ends. That means many of the people crossing the border do so in such a place. Knowledge of this, and the geography of the mountains there, allows agents to frequently think ahead to where smugglers and migrants might be headed.

Still, the amount of open territory in the area is vast, meaning that savvy crossers can still elude the Border Patrol from the get-go.