Building a drug-smuggling tunnel underneath the border isn't so tough, but finding the right place to do it is, experts say. Mexican cartels have figured out just the perfect spot along the border in terms of soil type, access and camouflage, and will likely continue.
Last week, U.S. authorities working with their Mexican counterparts discovered a 612-yard tunnel connecting two warehouses between San Diego and Tijuana.
The tunnel included electric rail cars to ferry pot between the two nations, as well as a sophisticated hydraulic system. Six people have been arrested in connection with the operation and authorities confiscated 16.7 tons of marijuana, the largest haul ever.
"The construction itself wasn't too challenging because you are underground, but the more challenging things are to be discreet so people don't find out," said Joe Garcia, special-agent-in-charge of the Immigration and Customs Enforcement agency and head of the San Diego Drug Tunnel Task Force.
Garcia says the tunnel was built by laborers using pneumatic shovels, jackhammers and old-fashioned sweat.
"You don't have ability to come in with a backhoe and start digging," he said.
The tunnel was only four to five feet high and about five feet wide, Garcia said. That's similar to a mining shaft. The men, probably a half dozen or so, slept inside a hidden room beneath the warehouse on the Mexican side, where law enforcement officials found blankets, food, microwave ovens and a small fridge. The men likely didn't leave the warehouse for weeks at a time.
The soil, or tailings, from the tunnel were sandbagged and stored inside the Mexican warehouse, Garcia said. The area of San Diego where the tunnel was found, near the Otay Mesa border crossing, consists of clay soils that form a good structure for the tunnel.
Any further west and the smugglers run into sandy soils near the ocean, any further west, and they hit the rocky mountains. While the Otay Mesa area is relatively small, it also happens to have a lot of legitimate warehouses used to store goods that are manufactured in Tijuana's maquiladora assembly plants.
From Otay Mesa, California, these goods are shipped around the United States. The whole industrial zone is a perfect cover for drug smugglers, and in fact the recent tunnel was the seventh sophisticated operation discovered in the past five years.
Years ago, U.S. homeland security officials were hoping to spot these tunnels from the air, using ground-penetrating radar aboard low-flying aircraft or drones. That didn't work out so well, Garcia said.
"Starting around 2006, we had a lot of technology come at us saying they could do this or do that," Garcia said. "They couldn't even detect a known site. There's nothing that is 100 percent accurate. The only thing that works is gold old fashioned investigative work and human intelligence."
Ground-penetrating radar only works well in dry soils, and is blocked by underground aquifers or wet, clay soils.
Tunnel expert Nick Sitar says there may be one answer to busting the cartels -- "geo-phones" that can detect an electrical current underground. The smugglers use electricity to run their digging equipment, lights and the rail cars.
The current generates a small magnetic field that can be detected by a device similar to a magnetometer. This kind of technology is often used by mining engineers to detect naturally-occurring currents running through subterranean formations, said Sitar, a professor of geo-engineering at the University of California, Berkeley.
Similar technology is being used to detect tunnels running under the Demilitarized Zone, or DMZ between North and South Korea.
"It's a huge cat-and-mouse game," Sitar said about the cross-border DMZ tunnels.
Unfortunately, underground electrical signals can be masked by other sorts of electrical conduits, such as power lines or gas lines that run underground in the Otay Mesa area. So as long as there are two urban areas next to each other along the U.S.-Mexico border, tunnel building will likely continue.
What about the cartel tunnel mastermind? He or she is likely a trained engineer, probably from one of Mexico's top mining schools and perhaps working in the copper mines in the Mexican state of Durango, said Garcia.
Sitar said the huge financial incentives offered by the drug cartels -- which are usually not optional -- outweigh any potential risks of getting caught. While the smugglers, or mules, were taken into custody, the engineer is still out there.
"I would guess that there are plenty of miners willing to make a lot more money making a tunnel for drug traffickers than they will in the mine," Sitar said.