Mexican troops stand guard at a "major cross-border drug tunnel" running from Mexico to California.
The tunnel links warehouses in an industrial park south of San Diego, and the Mexican border city of Tijuana.
Measuring around 370 metres, the tunnel has structural supports, electricity, and ventilation.
U.S. federal agents made the discovery after stopping a cargo truck seen leaving a warehouse.
They found three tons of marijuana in the vehicle, and another 6.5 tons in the warehouse.
Mexican authorities found another eight tons on the south side of the tunnel, much of it branded with Captain America labels.
A Mexican general said the tunnel was operated by the powerful Sinaloa cartel.
Tijuana is the principal gateway for drugs entering California from Mexico.
Drug cartels have dug scores of tunnels under the U.S.-Mexico border in recent years.
Nick Rowlands, Reuters.