The fences haven’t stopped the burros, who use either ropes or their bare hands to scale them. This was captured in extraordinary footage from a Mexican TV crew, showing smugglers climbing into Arizona. But solid walls offer no solution, as they can also be scaled and they make it harder for border patrol agents to spot what smugglers are up to on the Mexican side.

Flaco quickly graduated to building secret compartments in cars. Called clavos, they are fixed into gas tanks, on dashboards, on roofs. The cars, known by customs agents as trap cars, then drive right through the ports of entry. In fact, while most marijuana is caught in the desert, harder drugs such as heroin are far more likely to go over the bridge.

When customs agents learned to look for the switches that opened the secret compartments, smugglers figured out how to do without them. Some new trap cars can be opened only with complex procedures, such as when the driver is in the seat, all doors are closed, the defroster is turned on and a special card is swiped.

Equally sophisticated engineering goes into the tunnels that turn the border into a block of Swiss cheese. Between 1990 and 2016, 224 tunnels were discovered, some with air vents, rails and electric lights. While the drug lord Joaquin Guzman, known as El Chapo, became infamous for using them, Flaco says they are as old as the border itself and began as natural underground rivers.

Tunnels are particularly popular in Nogales, where Mexican federal agents regularly seize houses near the border for having them. Flaco even shows me a filled-in passage that started inside a graveyard tomb. “It’s because Nogales is one of the few border towns that is urbanized right up to the line,” explains Mayor David Cuauhtémoc Galindo. “There are houses that are on both sides of the border at a very short distance,” making it easy to tunnel from one to the other.

Nogales is also connected to its neighbor across the border in Arizona, also called Nogales, by a common drainage system. It cannot be blocked, because the ground slopes downward from Mexico to the United States. Police officers took me into the drainage system and showed me several smuggling tunnels that had been burrowed off it. They had been filled in with concrete, but the officers warned that smugglers could be lurking around to make new ones and that I should hit the ground if we ran into any.