The sophisticated 600m tunnel – equipped with reinforced walls, lighting, a hydraulic lift, electric rail carts and a wooden staircase – linked warehouses in Tijuana, Mexico, and San Diego, in California.
17 tons of weed were seized at a warehouse in San Diego, 11 tons in a truck in LA and 4 tons in Mexico.
The bust, which came after authorities began investigating in June, marks a seasonal trend.
For the last three years, authorities have found tunnels like these under the US-Mexico border at this time of year – to transport part of Mexico’s Autumn marijuana harvest into the US.
This year’s operation marks the second largest marijuana bust, according to the US Drug Enforcement Administration.
Special agent Derek Benner said: “We invite the cartels to continue in their futile efforts to smuggle marijuana into the U.S.”
The tunnel, which is 15ft (4.5m) deep, was one of the most sophisticated drug-smuggling tunnels found on the US border with Mexico.
A US police spokeswoman said: “It is clearly the most sophisticated tunnel that we have found in the last five years, perhaps ever.”
Six men were charged in Southern California’s federal courts with conspiracy to distribute marijuana.
Authorities in the US and Mexico have not linked Tuesday’s find to a particular cartel, but last year’s was linked to Mexico’s Sinaloa cartel, led by Joaquin “El Chapo” Guzman, the country’s most wanted drug lord.
