TIJUANA, Mexico – When smuggling goes smoothly for the marijuana division of the huge Sinaloa Cartel, cross-border deliveries unfold with clockwork precision.

Harvested marijuana arrives in plastic-wrapped bales at a depot hidden among the rundown warehouses on the Mexican side of the concrete U.S. border fence.

Once enough marijuana is collected, workers drop the vacuum-packed bales through shafts leading to the ever-more-elaborate tunnels that cross underneath the border through the clay-laden soil.

U.S. agents have been waging war against the tunnels for years. They use a range of high-tech devices, from ground-penetrating radar to seismic sensors, to find and destroy them. But despite the efforts, drug smugglers continue to build the tunnels, often spending $1 million to dig a single pathway equipped with lighting, forced-air ventilation, water pumps, shoring on walls and hydraulic elevators.

Lately, new tunnels have included railways. The bales move on electric mining carts with hand throttles that roll at up to 15 mph.

"A tunnel represents an incursion into the U.S., and it's a national security event," said Jose M. Garcia, who oversees the federal multi-agency San Diego Tunnel Task Force.

The location of the tunnels helps explain why agents have such difficulty finding them. The area where the most advanced tunnels have been found is adjacent to the Tijuana international airport, where scores of planes take off and land daily.

Nearby warehouses buzz with legitimate activity.

"All that noise from the airport is a great advantage to them," said Victor Clark Alfaro, an anthropologist and human rights activist in Tijuana who also lectures at San Diego State University. "This border is perforated like an anthill."

Soil makes digging easy

U.S. officials say they have found more than 160 tunnels since 1990 along the 1,954-mile border, mostly in the stretch of Mexico that borders Arizona and California. In the past 15 months, U.S. agents have busted increasingly sophisticated tunnels.

Geography and geology make the intensely urban Tijuana-San Diego corridor ideal for the tunnels. Tijuana is Mexico's sixth-largest city, with 1.3 million people, while San Diego is the eighth-largest U.S. city, with several interstate highways.

Moreover, soil here has a composition that's easy to dig.

In a two-week span last November, U.S. agents shut down two sophisticated tunnels that led from an area near Tijuana's airport to the Otay Mesa industrial park on the U.S. side. Some 49 tons of marijuana was seized. The discoveries marked the second year in a row in which elaborate tunnels were found within a mile of the busy Otay Mesa border crossing.

U.S. officials are sensitive about a public view that they aren't finding the tunnels.

"Understandably, American citizens react to news stories about the discovery of a large tunnel, complete with plumbing, lights, ventilation and a rudimentary railway system, with a mixture of surprise, indignation, alarm and dismay," Laura E. Duffy, the U.S. attorney for the Southern District of California, told the Senate drug caucus last June. "How, they ask, can such a sophisticated illegal structure be constructed right under our noses?"

Part of the difficulty, she said, is that drug traffickers use horizontal drills that cost up to $75,000 and can cut without disturbing topsoil. The tunnels run anywhere from 30 to 90 feet deep, avoiding greater depths, which would hit underground water tables.

Drug traffickers also have been adept at setting up bogus U.S. companies to rent space in bustling Otay Mesa and its 600 warehouses and 12,000 businesses. Many firms are unaware of activities by their neighbors, perhaps noticing only if there's truck traffic at unusual hours.

Garcia said that even with devices such as seismic sensors, a majority of tunnel busts came from tipoffs by informants or suspicious warehouse operators.

Mining experts enlisted

Big tunnels are thought to be the work of the Sinaloa Cartel, which has seized control of Tijuana from the local Arellano-Felix drug ring after years of bloody conflict and now is operating in tandem with remnants of the group.

Sinaloa operatives employ mining engineers and architects to help construct their tunnels, while keeping knowledge of locations to as few people as possible.

Experts on the San Diego Tunnel Task Force say "some tunnel excavators in Mexico are killed when the job is done to prevent them from spreading the word on the location," Duffy told senators.

Marijuana growers are turning to ever-larger plantations to meet the capacity of bigger tunnels. Last July, soldiers found a 300-acre screened and irrigated marijuana plantation – four times larger than any previously seized site – near San Quintin, 150 miles south of Tijuana. Eight months earlier, soldiers seized 148 tons of pot in Tijuana, a record.

Digging day and night

U.S. and Mexican agents say that tunnel digging, using pneumatic spades, generally is limited to teams of six or seven men. They live at the Tijuana site where the tunnel begins, and excavation is timed to conclude with the harvesting of marijuana crops in late summer and early autumn, so there's little time for the tunnel to sit idle and be detected.

"The process is tedious," Garcia said, involving working day and night and lugging bags of dirt along the shaft for removal.

But even with million-dollar investments, Garcia said, the tunnel builders "recoup that by making just one trip, given the value of the narcotics we've seized."

Most bales of marijuana carry stickers, often fanciful images such as Donald Duck, Captain America, Budweiser or Homer Simpson. The stickers indicate ownership and destination, U.S. agents said.

Tunnel operatives make sure to recoup their investments first.

"The way it works is the tunnel guys build it, so their stuff gets through first. Once it gets through, they start hiring out" to other drug organizations, said Louis Gomez, the supervisor of the San Diego Tunnel Task Force, which includes agents of Customs and Border Protection, the Drug Enforcement Administration, Immigration and Customs Enforcement and the California Bureau of Narcotic Enforcement.

Tunnel shafts on the Tijuana side that a McClatchy journalist visited included one hidden in the floor of a walk-in freezer in a warehouse that's only two football fields away from warehouses on the U.S. side of the border.

Another shaft was hidden in a unique fashion: "It was the entire floor of a bathroom that went up and down, and they used a hydraulic lift like you'd see in a service station," Garcia said.

Tijuana Police Chief Alberto Capella Ibarra said the tunnels kept growing in sophistication.

"It speaks of the strength and economic power of the cartels, because these tunnels are a huge investment for them," Capella said.

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