(CNN) -- The discovery of a "sophisticated" tunnel between a Mexican eat-in kitchen and two Southern California warehouses led authorities on both sides of the U.S.-Mexico border to arrest at least nine suspects and seize between 20 and 30 tons of marijuana.
The Thanksgiving Day find followed an eight-month investigation and came about three weeks after authorities discovered a similar drug tunnel in another warehouse in the Otay Mesa area of San Diego.
Between the two incidents, agents in the United States and Mexico seized roughly the equivalent of one marijuana cigarette for each of California's nearly 37 million residents, Drug Enforcement Administration special agent Ralph Partridge said Friday.
"This is obviously not a Mom and Pop operation," Miguel Unzueta, a San Diego-based agent with Immigration and Customs Enforcement, said Friday. "This is a major, significant drug cartel working."
Unzueta described the nearly half-mile-long and, in spots, 90-foot-deep passageway as "very sophisticated," even more so than the one found earlier this month.
Its southern entry was in the kitchen of a stucco Tijuana, Mexico, home, which had a garage attached big enough for tractor-trailer trucks. After removing a 2-foot by 4-foot piece of flooring in that house, smugglers could head 80 feet down a cinder-block-lined stretch into the tunnel.
The passageway had lighting, ventilation, wood and cinder-block supports, wood floors and rail carts, U.S. law enforcement agents said. The tunnel split to lead into two separate warehouses in San Diego.
"Having been in several of these tunnels, this is one of the most sophisticated ... I've ever seen," Partridge said. "This is a tunnel where a lot of drugs were being pushed through."
The discovery came after San Diego Tunnel Task Force agents, on Thursday morning, spotted a tractor-trailer arriving at one of the warehouses, said Unzueta. After getting its load, the truck headed to a border patrol checkpoint in Temecula, California, about 60 miles north of San Diego.
There, authorities who had been tracking the shipment found the truck filled "top to bottom, front to back" with 27,000 600-pound packages of marijuana, Unzueta said.
Authorities initially could not get through the tunnel from the California side because it was blocked by huge packages of marijuana. Much of the drug was in a 10- by 20-foot room about 60 feet below the surface. U.S. agents and Mexican military personnel formed a human chain to lug out what amounted to 3 to 4 tons of marijuana, according to Unzueta.
In addition, U.S. law enforcement agencies tipped Mexican authorities about a ranch on that side of the border, where 3 to 4 more tons of the drug was captured.
Unzueta estimated that all the marijuana seized Thursday could have sold wholesale for $17 million to $20 million or -- as Partridge estimated -- been equal to 16 million to 17 million individual 1-gram joints.
The driver of the tractor-trailer truck was arrested and, if convicted on federal drug smuggling charges, could face between 10 years to life in prison, said Assistant U.S. Attorney Alana Robinson.
The driver of a box truck who accompanied the tractor-trailer to the San Diego warehouse also was arrested, Unzueta said. Although that truck didn't carry any drugs, the residence it went to had $13,500 in cash. Both trucks' drivers are U.S. citizens and are now in federal custody.
Mexican authorities have arrested at least another seven people, Unzueta said.
Earlier this month, agents made several arrests after seizing about 30 tons of marijuana related to the other San Diego tunnel discovery. John Morton, the head of the U.S. immigration and customs agency, said authorities "caught them in the act." And in October, Mexican authorities seized 105 tons of marijuana in Tijuana.
Law enforcement agents on both sides of the border have found at least 75 cross-border tunnels in the past four years, most of them in California and Arizona, according to Unzueta.
"Internal drug traffickers are using transborder tunnels to import tons of marijuana into the United States," Robinson said. "Clearly, these transborder tunnels pose a significant threat to our national security as well."