Photo from U.S. Customs and Border Patrol.

Close to the border wall separating the town of Nogales, Ariz., from its much larger sister city, Nogales, Sonora, in Mexico, are abandoned stores and houses. Cameras and Border Patrol agents are only a few of the visible markers of the town’s preeminence as a smuggler’s Shangri-La. Another clue is that International Street, which runs along the wall, is devoid of parking meters. Before the uprooting of the meters, cars without floors, parked over one of the many entry points to the massive drainage system connecting the two towns, would wait as hands from below ground raised the package and another set of hands, inside the car, picked it up. Once completed, a hydraulic jack from below replaced the cement cover, smugglers in the car’s trunk glued the plug back in place with concrete sealant, and the car would wind its way out of town. When authorities finally notice one of these loose patches, the city’s repair crews fill it with fresh concrete and mark the spot with a date, so that they can keep track.

Some of them are hand-dug affairs, excavated by cartel-employed miners for months at a time. A tunnel, like the 481-foot one unearthed last month, may be but a passageway two feet wide and three feet tall with wood shoring, electric lighting and fans to circulate the air. Others are fitted with electric rail systems so that mining carts can ferry marijuana that will eventually arrive on America’s streets with a wholesale price of $400 a pound. The truly extravagant tunnels feature hydraulic doors, even elevators.

In the 1930s, well before the Drug War divided the twin cities, they occupied the same narrow bit of valley that ended in flood plain; the Mexican city sat on the higher ground while the U.S. city lay below. To accommodate the rain that washed downhill during rainy season, American engineers turned the arroyos in Nogales into large culverts that ran beneath the U.S. city’s two main streets, Morley and Grand Avenue. The endeavor created access to a web of underground corridors that has sheltered, at one time or another, cars, children, and cannabis.

Some tunnels cost as little as $30,000 while others as much as $1.5 million. And while the Border Patrol continues to discover these hidden transit routes, the underground has never been completely mapped and remains largely unknown. For some an ordeal; for others, an opportunity.