Nogales, Arizona (CNN) -- As night falls, U.S border patrol agents know smugglers will use the dark to sneak into the country with illegal immigrants and drugs -- both above and below ground.
Under the Arizona border city of Nogales is a network of tunnels -- some built as part of the city's flood defenses, others built by the smugglers.
U.S. Border Patrol agent Michael Camdron says: "We're usually trying to see some sign that someone's been coming through, pushing bundles of marijuana or something like that."
In the U.S. illegal immigration is a hot political potato and in Nogales it's a part of everyday life as immigrants use the area as a crossing point.
The Grand Tunnel was built right underneath the border crossing at Nogales to relieve both sides of the border during the monsoon season.
Agents get into the tunnel by lifting a heavy grate and climbing down a metal ladder. Once inside the tunnel, only flashlights separate them from complete darkness as they inspect the mile-long structure.
Agent Ariel Medeles, who is in joint charge with Camdron of inspecting the tunnel, says they have various techniques to find out whether anybody has tampered with the tunnel's security.
