Thousands gather south of the Arizona border awaiting their turn to enter the country and request asylum, but still others look for areas along the border where they can step onto U.S. soil. Now the U.S. government is rushing to build dozens of miles of new border fence. It was a campaign promise in far-flung Washington, D.C., four years ago. It’s happening now in Arizona. → Special Report: 'Tracing The Migrant Journey'