Drug smugglers trying to tap into the United States’ highly profitable market have developed all kinds of secretive routes to deliver their illicit goods. One tactic is to go underground.

At the U.S.-Mexico border, tunnels are a popular means of transporting illegal marijuana – a bulky drug whose distinctive odor makes it hard to get past drug-sniffing dogs and other security at border checkpoints and airports. Since 1990, more than 220 tunnels have been discovered there, including 60 in the area overseen by the U.S. Border Patrol’s San Diego office.

“You see, it’s an industrial area,” agency spokesman Jose Hernandez says in explaining its attraction to traffickers.

“We see many warehouses and many trucks passing by. It is common to see them at two in the afternoon or the morning,” he adds. “It happens many times. So what [traffickers] do is they hide where everyone can see them anyway.”

Noise from constant truck traffic on the American side and dense urban construction on the Mexican side conceal the construction of secret tunnels.

WATCH: Related video on border tunnels

Underground investment

Drug traffickers will spend as much as $2 million on construction in hopes of big profit in return, the Drug Enforcement Administration told the Associated Press earlier this year. The DEA links most of the cross-border tunneling to Mexico’s Sinaloa drug cartel, whose boss, Joaquin “El Chapo” Guzmán used other tunnels to escape twice from Mexican prisons. He was extradited to the United States in January and awaits trial on various trafficking charges.  

The unfinished Galvez tunnel, discovered in 2009, was intended as a link from about 18 meters south of the Mexican border stretching 231 meters north into California’s San Diego neighborhood of Otay Mesa.

From transport to training site

Now, instead of serving as a conduit for drugs, it has become a training site for Border Patrol agents. They scramble down ladders to a passageway 20 meters deep, nearly 2 meters tall and just over a meter wide.  

The tunnel yields clues into tunnel engineering and equipment. Dug with drills, picks and shovels, it was outfitted with lighting, railway, telephone and ventilation systems.

That’s not the most elaborate. Lance LeNoir, who leads the Border Patrol’s San Diego Intercept Team of so-called “tunnel rats,” reports that agents once “intercepted 13 individuals immediately to our south in Mexico. They found the entrance underneath a bathroom floor, which actually lowered through a [hydraulic] lift system.”

To aid its detective work, the Border Patrol uses an assortment of complex tools, including ground-penetrating radar and sensors.

Closing the link

When a tunnel is discovered, U.S. authorities shut it down.

“When we go to remediate these things, we mark the ground with an X,” LeNoir says, then “drill down to find them and fill them with cement.”

From 2007 to 2015, the Department of Homeland Security spent $8.7 million to block the tunnels with concrete.

Its counterpart in Mexico uses a different approach.

“They mostly put garbage at the entrance to the tunnel until the entrance is blocked,” says Hernandez, the Border Patrol spokesman.

The trash treatment is a source of conflict between Mexican and U.S. authorities because it’s not permanent. At least six tunnels were found reopened on the Mexican side. Authorities there say they don’t have the money to use cement.

But the two countries continue to collaborate on blocking tunnels and traffickers. The United States views illegal drugs and smuggling as a threat to the nation’s health and security. 

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