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SAN DIEGO — A drug-smuggling tunnel believed to be the longest one ever has been discovered underneath the U.S. and Mexico, federal authorities announced Wednesday.

The narrow passageway runs for nearly a half-mile between Otay Mesa, Calif., and Tijuana, Mexico,and is outfitted with lights, a ventilation system and an elevator.

Tunnel elevator leading to the surface. (U.S. Attorney’s Office Southern District of California)

An observant Border Patrol agent’s suspicions led to a roughly eight-month investigation that ended last week with six arrests and the discovery of the tunnel.

The subterranean passageway connects a run-down home in Baja California to a fenced-off lot filled with wooden pallets in the far southern reaches of San Diego, according to KSWB.

The north end of the tunnel was hidden by a “jumbo-sized” commercial trash bin that doubled as a container for narcotics spirited into the United States, U.S. Attorney Laura Duffy told reporters during a midday briefing.

A small opening hidden by gravel spread over the lid — “literally a hole in the ground” — led to the roughly 10-foot-deep, three-foot-diameter zigzagging underground channel, in which agents found 68 packages of marijuana weighing a combined 1,600 pounds, Duffy said.

Inside the 800-yard tunnel being called the longest ever found between Mexico and California. (U.S. Attorney’s Office Southern District of California)

Three people were arrested at the storage property and another three were taken into custody at a parking lot on Imperial Avenue, near 30th Street in San Diego, where traffickers had taken the garbage bin and partially unloaded its cargo of drugs into a truck.

In the vehicle, agents found about 11,000 pounds of cannabis and 2,242 pounds of cocaine. The cocaine seizure is believed to be the largest ever made in California in connection with a smuggling tunnel, according to Duffy. In the commercial trash container was another 1,430 pounds of marijuana, she said.

Agent clears rocks away from a wood plank covering the tunnel opening. (U.S. Attorney’s Office Southern District of California)

Investigators said the thwarted delivery was the first real smuggling attempt after a series of “dry runs” observed by agents who had the pallet-filled lot under surveillance for months, the U.S. attorney said.

The southern end of the passageway emerges into a commercial- grade elevator concealed in a closet in a Tijuana “flophouse” with mattresses strewn about inside, Duffy said during the news conference at the Otay Mesa lot, where a sign reading “Otay Pallets — We Buy Pallets” was posted on the perimeter fence.

The discovery came six months after U.S. and Mexican investigators ferreted out a well-appointed tunnel stretching almost as far underneath the international line between warehouses in the same two border communities.

Law enforcement agents raided that roughly 800-yard-long passageway — which was outfitted with ventilation, electric lighting and rails for ferrying contraband — on the night of Oct. 21, hours after it went into use for the first time, authorities said at the time.