The cartels often rely on forced labor to dig the tunnels. Poor Mexicans, frequently from the interior of the country, have been known to take work offers from cartel operators, only to find themselves locked in warehouses, working in shifts to dig north.
A view of the US entrance to a 400-yard cross-border tunnel after authorities recovered 14 tons of marijuana from warehouses in Otay Mesa industrial park and Tijuana, Mexico, in Otay Mesa, California, November 16, 2011. Police discovered the tunnel running to California from Mexico, and seized more than 17 tons of marijuana, US and Mexican authorities said.