The secret passage from a warehouse in Tijuana to an industrial area in San Diego featured rail and ventilation systems, electricity and reinforced walls.
It was discovered near San Diego’s Otay Mesa border crossing in an area where more than a dozen other sophisticated tunnels have been found in the last two decades.
US authorities said it was unknown how long the tunnel had been operating and what amount of drugs, if any, got through undetected.
US authorities hold a press conference after the drug seizures and show clips of the tunnel
They seized 799kg of cocaine, 75kg of meth and 1.6kg of heroin in connection with the investigation.
Six people, ages 31 to 55, were charged with conspiring to distribute cocaine. All lived in Southern California.
The tunnel goes directly beneath one of the most heavily guarded parts of the US-Mexico border.
Walls that go deep into the ground cover most of the border in the region, often blocking smaller crudely built tunnels, but cannot prevent more sophisticated passages like the one discovered this week.
This tunnel was six stories below ground, around 4ft in diameter and ran for one-third of a mile.
The tunnel exited the United States in a nondescript warehouse named "Amistad Park" on a street that is busy with large semitrailers during the day but quiet at night.
On Monday, armed guards watched over a small shaft with a ladder that descended into the tunnel.
After staking out a home that was recently used to stash drugs, officials began making traffic stops of vehicles that had been there or at a warehouse near the border, turning up boxes full of cocaine, according to a federal criminal complaint filed in San Diego.
They raided the properties — finding no other drugs at the warehouse, but a tunnel opening carved into the cement floor, federal prosecutors said.
Authorities have found about 15 sophisticated tunnels on California’s border with Mexico since 2006.
“There is no more light at the end of this narco-tunnel,” said US attorney for the Southern District of California Randy Grossman.
"We will take down every subterranean smuggling route we find to keep illicit drugs from reaching our streets and destroying our families and communities."
By federal law, US authorities must fill the US side of tunnels with concrete after they are discovered.