By Marty Graham

SAN DIEGO (Reuters) – Federal agents have seized a ton of cocaine and seven tons of marijuana smuggled through a clandestine tunnel stretching a half mile beneath the U.S.-Mexico border, the longest one yet unearthed in California, authorities said on Wednesday.

Six people were arrested as authorities in San Diego moved on Monday and Tuesday to shut down the tunnel, the 13th underground passageway discovered along California’s border with Mexico since 2006.

The 870-yard-long tunnel, one of the narrowest found in the region, also yielded an unprecedented cache of drugs.

“This is the largest cocaine seizure ever associated with a tunnel,” said Laura Duffy, U.S. Attorney for the Southern District of California.

The northern end of the tunnel, like most of the others, emerges in a narrow industrial expanse between the Otay Mesa port of entry and the California Highway Patrol’s border facility. The area, known for its heavy clay soil, is primarily traversed by trucks hauling tons of legitimate cargo between the two countries every day.

The latest tunnel, excavated 46 feet beneath the surface, ran from the bottom of an elevator shaft built into a house in Tijuana to a hole in the ground on the U.S. side enclosed within a fenced-in lot set up as a pallet business. The hole was concealed under a trailer-sized trash dumpster that smugglers used to move the drugs off the lot, federal officials said.

“They put the drugs in the dumpster and then hauled the dumpster to another location to unload it,” Duffy said. Federal agents followed a truck that carted the dumpster to a central San Diego spot about 25 miles north of the border and watched as the cargo was loaded onto a box truck, which drove away.

San Diego County sheriff’s deputies who stopped the truck seized 2,242 pounds of cocaine and 11,030 pounds of marijuana, and arrested three men, Duffy said. Federal agents searching the pallet lot and the tunnel recovered an additional 3,000-plus pounds of marijuana and arrested three more suspects, she said.

The suspects were all jailed on various drug-trafficking conspiracy charges.

Federal agents who patrol the Otay Mesa area immediately north of the border began watching the pallet company, its yard stacked with grimy, wood-frame racks, in October, Duffy said.

“The investigation began with an astute border patrol agent who identified this business as suspicious,” Duffy said. “They began monitoring this location and saw the people here conducting dry runs.”

(Editing by Steve Gorman and Andrew Hay)