S3, E4: Cartels and illegal immigrants have found weak spots along the U.S.- Mexico border. Four feet beneath Nogales, Ariz., agents crawl through a claustrophobic drug-smuggling tunnel, knowing they could come face to face with heavily armed criminals. Trains, with a myriad of hiding places, are also becoming a favorite place for cartels to stash drugs. If the officers aren't careful, they could easily miss a load.