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The half-mile passage runs from a residence in Tijuana to a warehouse in San Diego's Otay Mesa area, the San Diego Tunnel Task Force said in a statement.
A Mexican man linked to one of the largest marijuana seizures has pleaded guilty to drug charges.
Daniel Navarro entered his plea Tuesday in federal court in San Diego, admitting to his role in one of the most elaborate drug smuggling operations in U.S. history.
Prosecutors say Navarro oversaw the transport of 50 tons of marijuana through two sophisticated cross-border underground tunnels into California.
The tunnels and drug loads were discovered Nov. 2 and Nov. 25. One tunnel spanned nearly seven football fields in to Mexico investigators said.
The tunnel connected an Otay Mesa office complex and warehouse to another warehouse in Tijuana. It had lighting, ventilation and a rail system for sending carts of drugs into the United States.
The tunnel ended close to California's Otay Mesa port of entry, according to U.S. Immigration and Customs Enforcement spokesperson Lauren Mack.
Federal agents found it after getting a tip that there was one somewhere on Via De La Amistad. After they spotted a suspicious looking tractor-trailer parked outside an office building, they followed the truck as it headed north on Interstate 15. When it reached the U.S. Customs and Border Protection checkpoint near Temecula, agents searched it and found 10 tons of marijuana.
When agents served a search warrant at the tunnel location, they seized about 22 tons of pot estimated to be worth more than $100 million.
The driver Carlos Cunningham, Jr., 31, of Oceanside, admitted to hauling multiple loads of marijuana from that building to other areas of California. He was sentenced in April to 21 years in prison.
Navarro faces a maximum penalty of life in prison and a minimum sentence of 10 years. He will be sentenced Aug. 19.
Officials have found 125 tunnels since they started recording them in the early 1990s. Of those, 75 have been found in the past four years, most along the California and Arizona border with Mexico.