The Sinaloa cartel developed its tunnel-building technique rapidly. Soon Guzmán's organization was constructing massive passages, hundreds of meters long, equipped with electricity, ventilation, and transport systems. "They've got skilled engineers making these, people who are qualified engineers, who will reinforce that tunnel, make it big, and have it so you bring actually rails on them, with trains, electric lights, air vents — very important, the air vents," Gillo told Business Insider.