U.S. Border Patrol agent Richard Funke looks for footprints from illegal immigrants crossing the U.S.- Mexico border on December 7, 2010 near Nogales, Arizona. Although a new fence has been built along the majority of Arizona's border with Mexico, critics have called for fencing of the entire stretch. Much of the unfenced terrain is in remote and mountainous areas. Although illegal immigration has slowed all along the U.S. Mexico border, Border Patrol officials say the Tucson sector remains the most heavily trafficked in the nation. (Image source: Getty Images)
