Los Angeles , California ( CNN ) -- U.S. authorities have discovered about 30 tons of marijuana that were part of a smuggling operation using a tunnel under the California-Mexico border , officials said Wednesday . The 600-yard tunnel -- which features a rail system , lighting and ventilation -- connects a warehouse in Tijuana with one in the Otay Mesa industrial area of San Diego , said U.S. Immigration and Customs Enforcement spokeswoman Lauren Mack . About 26 tons of marijuana had been transported through the tunnel to San Diego , and 10 of those tons were intercepted Tuesday by authorities as a tractor trailer was transporting the load from the Otay Mesa warehouse , officials said . About five tons were found by the Mexican military inside the Tijuana warehouse and the tunnel , officials said . Drug cartels on the border have become so powerful and sophisticated in recent years that many Mexican communities and areas along the border are patches of uncontrollable violence , experts have said . `` It 's not a good day for the cartels , '' said ICE director John Morton . `` They now ca n't move that size of drugs without digging a tunnel for 600 yards . It backfired on them . `` Obviously this is a cartel and organized drug smuggling of the highest order , '' Morton added . Authorities were n't able to identify Wednesday which cartel was behind the tunnel operation , he said . The smuggling was active for about a month until this week 's seizure . The tunnel was rather small , and an individual ca n't stand up in it , Morton said . He described the railway as `` crude . '' The seizure was also unusual because authorities made their bust while the smuggling was active , Morton said . `` We caught them in the act , '' Morton said . `` We find these tunnels and they 're usually abandoned . '' The seizure was also one of the largest on the California-Mexican border , officials said . `` What 's unusual about this one is the amount of marijuana found as part of this investigation , '' Mack said . The 30 tons is considered significant by U.S. and Mexican authorities even though Mexican authorities seized 105 tons of marijuana in Tijuana last month , the largest Mexican bust in recent years , Mack said . `` So there 's been some pretty big drug busts , '' she said . `` We 're not letting our guard down . '' In the past four years , 75 smuggling tunnels have been discovered on the U.S.-Mexican border , most of them in California and Arizona , authorities said . In all , about 125 tunnels have been found since the early 1990s , when authorities began keeping count , with just one of them on the U.S.-Canadian border , Mack said . Authorities will be investigating the owners of the Tijuana and San Diego warehouses , officials said . A special U.S. border tunnel task force hunts for underground smuggling operations in and around San Diego . The task force consists of agents from ICE , the U.S. Drug Enforcement Administration and the U.S. Border Patrol , and they also work with Mexican authorities , Mack said . In its surveillance of the Otay Mesa warehouse , the task force noticed suspicious activity Tuesday when they saw a tractor-trailer leave the facility . Agents followed the truck to a Border Patrol checkpoint at Temecula , California , and authorities found 10 tons of marijuana hidden in cardboard boxes on pallets , said ICE director John Morton . A married couple driving the truck was arrested , he said . The task force became the first of its kind in 2003 , when it was assembled to deal with a growing number of underground smuggling routes on the California-Mexico border . The unit was also assembled as part of a post-September 11 security concerns , Mack said . The longest tunnel discovered , found in 2006 , had a length of seven football fields . That tunnel also connected warehouses in Otay Mesa and Tijuana . The task force uses robots to scout out a newly discovered tunnel before agents are sent into it . Federal agents are trained like miners on how to negotiate confined spaces , and the San Diego-Mexico region is even used to test the latest ground-penetrating technology to detect tunnels , including by the U.S. military , Mack said . The sophisticated tunnels -- with lighting , oxygen pumps and rail lines -- are typically used to ferry drugs from Mexico to the United States . The more rudimentary tunnels are just big enough to smuggle people into the country , Mack said . `` We 've also been enjoying an unprecedented cooperation with Mexican law enforcement in recent years , '' Mack said . `` So we get a lot of information from the Mexicans , and vice versa . ''
NEW : ` ` It 's not a good day for the FBI , '' says ICE director John Morton . NEW : Smugglers were caught in the act , Morton says . A 600-yard tunnel connected offices in Tijuana and San Diego , officials say . 26 tons of smuggled drugs were found on the US border and 4 in Mexico .