FBI special agent Eric Drickerfen inspects metal pipes from a 600-foot long network of tunnels hidden underneath the international border. Because of the high cost of tunnel construction, their financing is usually the work of drug cartels. Agent Mandel searches for illegal immigrants submerged in New River, a polluted waterway that crosses the international border. Julia Tellez places a flower at the grave of an unknown immigrant who died while attempting to cross the border from Mexico into the US. The graveyard bears dozens of crosses that read 'No Olvidado' or 'Not Forgotten.' Agents from the US Border Patrol chat along the border fence in the desert near Yuma, Arizona. With long stretches of waiting in between tracking and chasing immmigrants, boredom is a part of law enforcement along the border. Sewage and industrial effluent clogs New River and presses against the border fence, as it flows from Mexico into Calexico, California. A seasonal farm worker from Mexico harvests an artichoke farm in nearby Brawley, California. Although the wages of most seasonal workers exceed the California minimum wage, in Mexico their income exceeds the average per capita income. (Source: U.S. Dept. of Labor). Border Patrol agents Quinn Palmer and Barbara Mandel surveil the border between Calexico, California and Mexicali, Mexico under the cover of shadows. In a throwback to the 19th century, a horse patrol guards the border at night. Horses are valued for their silence and ability to traverse rough terrain. In a well-known tactic to distract the Border Patrol, human smugglers light tires on fire on the Mexican side of the fence. As agents approach the fire, illegal immigrants hiding elsewhere make a break for the U.S. border. A search dog leads agent Angel Hernandez to a truck yard, where he finds two men hiding inside a mobile home. They surrender without resistance. The immigrants are transported to a detention facility in nearby El Centro, California. Charged with a misdemeanor, they'll likely be returned to Mexico without punishment. Illegal immigrants leave behind a reminder of their crossing: a shoe stuck on the top of the American side of the border fence, suspended about 10 feet above the sidewalk. These 'fence fashions' left by illegal immigrants - gloves, hats, shoes - line the border fence for miles around the city of Calexico.