San Diego is “a candy store for smugglers,” a Tunnel Task Force agent said. Illustration by Paul Rogers

At 8:52 P.M. on July 11th, Joaquín Guzmán Loera, the drug kingpin known as El Chapo, sat on the bed of his cell in Altiplano, Mexico’s only super-maximum-security prison. Surveillance footage appears to show a small screen glowing on a table nearby—inmates are not allowed cell phones, but this rule is not always enforced. Guzmán changed his shoes, walked to a shower area in the corner of the cell, and knelt behind a waist-high concrete partition, out of view of security cameras. Six seconds later, he was gone.

A rough-edged opening, about twenty inches square, had been cut into the floor. According to Mexico’s national-security commissioner, Guzmán climbed into the hole and down a ladder, entering a 4,921-foot-long tunnel. Fluorescent lights hung from a ceiling-mounted PVC pipe, which also brought fresh air into the passageway. Metal tracks had been bolted to the ground, allowing an ad-hoc vehicle—a railcar rigged to the frame of a small motorcycle—to be driven from one end of the tunnel to the other. The gray stone walls, about thirty inches apart, were scored with jagged marks made by electric spades; Guzmán’s shoulders probably brushed the walls as he passed.

The tunnel ended beneath a small cinder-block house in an open field. As Guzmán climbed a wooden ladder toward ground level, he passed the evidence of what seemed to be a months-long engineering project: a generator, which had powered the tools that workmen used to build the tunnel; a heavy-duty electric winch, to lower machinery into the pit; gallons of hydraulic fluid; coils of steel mesh.

Guzmán’s method of escape should have surprised no one. Last year, in Culiacán, he evaded Mexican marines by disappearing into a network of subterranean passageways connecting seven houses. He did not invent smuggling tunnels—bank robbers, rumrunners, and guerrillas had used them for decades—but his criminal enterprise, the Sinaloa drug cartel, built the first cross-border narcotúnel, in 1989. Since then, Sinaloa has refined the art of underground construction and has used tunnels more effectively than any criminal group in history.

In the past quarter century, officials have discovered a hundred and eighty-one illicit passages under the U.S.-Mexico border. Most have been short, narrow “gopher holes” just big enough for a person to crawl through. Sinaloa specializes instead in infrastructural marvels that federal agents call supertunnels. Agents estimate that a single supertunnel takes several months and more than a million dollars to build. Many include elevators, electric lights, ventilation ducts, and cleverly disguised entry and exit shafts. They can reach as deep as seventy feet, and they tend to be tall enough for an adult to walk or ride through.

These days, most of Sinaloa’s supertunnels are used to ferry drugs across the border, from Garita de Otay, an industrial neighborhood in northern Tijuana, to Otay Mesa, a similar area in southern San Diego. Otay Mesa, which is bounded on the north by Brown Field Municipal Airport and on the south by Mexico, consists of highways, strip malls, and a few hundred warehouses clustered near the border. Most supertunnels terminate inside these warehouses, making them difficult to detect.

The amount of warehouse space in Otay Mesa has nearly quadrupled since the mid-nineties, and the expansion has been almost as frenetic in Garita de Otay. Forklifts, jackhammers, and heavy vehicles attract little attention. Cartel trucks back into loading bays, pallets are loaded in, and the drugs are delivered north to distribution hubs. There are three official border crossings near Otay Mesa; one, for commercial vehicles, is inside the industrial zone. “All of this has created a candy store for smugglers,” a U.S. agent told me. “This whole area belongs to them.”

Hundreds of federal agents—from Border Patrol, Homeland Security Investigations, the Drug Enforcement Administration, and Immigration and Customs Enforcement—work in a pair of large unmarked buildings on the edge of the Otay Mesa district. Among them are the ten members of the San Diego Tunnel Task Force, a multi-agency group created in 2003. The agents have found an average of two tunnels a year, but most of the people they have arrested have been low-tier Sinaloa operatives such as truck drivers and warehouse supervisors. Information within the cartel is compartmentalized, so that even when workers are caught and tempted with plea bargains they are unable to divulge much actionable intelligence.

“There are so many questions,” Tim Durst, a former Tunnel Task Force supervisor, told the Wall Street Journal, in 2013. “What are their techniques? How the heck do they build these things so well?”

Recent investigations—including a pending case involving a man believed to have been Sinaloa’s highest-ranking tunnel manager—have provided some answers. Sherri Hobson, a federal prosecutor in San Diego, told me, “I think it’s a very small group of élite members of the cartel that are doing this. This is highly sophisticated work. A lot of people think that you have a shovel and you dig. That’s not the way it works.”

In December of 2012, a nineteen- year-old named Fernando walked into Mama Mia, a pizzeria in a Tijuana strip mall, and asked for a job application. As he filled out the form, a stranger entered the shop. According to statements later collected by Mexican authorities, the man handed Fernando his phone number and asked whether he wanted a job cleaning a convenience store.

Fernando never heard back from Mama Mia. Eventually, desperate for work, he called the stranger’s number and met him at the strip mall. The man offered good money—twelve hundred pesos (about seventy-five dollars) a week—and Fernando agreed to go with him to look at the job site. From the strip mall, a highway leads north, past the graffiti-covered concrete walls surrounding the Tijuana Airport to the pitted roads of Garita de Otay, where convoys of eighteen-wheelers stir up dust that never quite settles. The warehouses, bland and beige, resemble cardboard boxes.

They stopped in front of a structure with no identifying marks except the street address, stencilled in black. Inside, behind a rolling gate, was a loading bay big enough to accommodate a dump truck. Inside was a storage room with cinder-block walls. Fernando didn’t see anyone else in the storage room—just a deep hole and sacks of dirt. The man told Fernando that things had changed: he would be digging a tunnel, not cleaning a store. If he tried to leave, he and his family would be killed.

Around that time, sixteen other men fell into the same trap. Across Tijuana, at bus stations and on busy street corners, they were lured to the warehouse by the prospect of temporary jobs. Some said that they had been promised safe passage across the border in exchange for a few hours of construction work. Fernando was the youngest of them, and one of only two Tijuana natives. Most were laborers from Mexico’s rural interior who had travelled north seeking opportunity.

According to the men, the overseer of the project, who called himself Carlos, was in his mid-thirties, with a thin, weedy mustache and a baseball cap pulled low over his brow. Carlos split the men into two groups. Fernando worked the day shift, from 6 A.M. to 6 P.M.; at night, he slept in the warehouse with the rest of his crew. Carlos brought the workers food and made sure no one left the building.