The 600-yard passage linking warehouses in San Diego and Tijuana included a wooden staircase, lighting and ventilation, said Derek Benner, U.S. Immigration and Customs Enforcement's special agent in charge of investigations in San Diego. It was tall and wide enough to move comfortably inside.
"This is an incredibly efficient tunnel designed to move a lot of narcotics," Benner told The Associated Press.
Authorities recovered nearly 17 tons of marijuana at the warehouse in San Diego's Otay Mesa area, nearly 12 tons inside a truck in Los Angeles and about 4 tons in Mexico, Benner said. Several arrests were made.
Tuesday's find was the latest in a spate of secret passages found to smuggle drugs underground from Mexico, a response to heightened enforcement on land. In an emerging seasonal trend, many are turning up shortly before the winter holidays in what authorities believe is an effort to take advantage of the Mexican harvest season.
Two weeks ago, authorities seized 17 tons of marijuana in connection with a tunnel linking warehouses in San Diego and Tijuana.
Raids last November on two tunnels linking warehouses in San Diego and Tijuana netted a combined 52 of marijuana on both sides of the border. Those secret passages were lined with rail tracks, and had lighting and ventilation.
Authorities believe the latest tunnel began operating recently but declined to provide details.
"I would say it's probably as sophisticated as any we've ever seen," William Sherman, acting special agent in charge of the U.S. Drug Enforcement Administration in San Diego told the AP.
More than 70 tunnels have been found on the border since October 2008, surpassing the number of discoveries in the previous six years. Many are clustered around San Diego, California's Imperial Valley and Nogales, Ariz.
California is popular because its clay-like soil is easy to dig with shovels. In Nogales, smugglers tap into vast underground drainage canals.
San Diego's Otay Mesa area has the added draw of plenty of warehouses on both sides of the border to conceal trucks getting loaded with drugs. Its streets hum with semitrailers by day and fall silent on nights and weekends.
Associated Press writer Mariana Martinez contributed to this report from Tijuana, Mexico.