Mexican authorities found the entrance to the 600 metre passage in a Tijuana warehouse, after a tip-off from US authorities.
At 15ft deep, the tunnel is equipped with hydraulic lifts as well as lighting and transport carts, showing the wealth and technical sophistication of the country’s most powerful drug cartels.
The other end of the tunnel was discovered in nearby San Ysidro in California.
US authorities said that 32 tonnes of marijuana, worth $65m, were confiscated following the operation. The US Drug Enforcement Administration said the discovery is the second biggest drugs bust in US history.
A passage of this scale takes roughly six months to a year to complete, authorities esimate. Workers used shovels and pickaxes to dig and often sleep in the warehouse until the job is finished.
It is the second tunnel to be discovered in the area this month, and authorities believe the work is an attempt to transport Mexico’s bumper autumn marijuana harvest before the winter holidays.
“You have an organisation that knows that they’re going to have X amount of inventory at a certain time of the year,” said US Immigration and Customs Enforcement Special Agent Derek Benner.
“Similar to legitimate businesses that monitor their inventory, and they want to find the most effective and efficient means to distribute that inventory and sell it as they can.”