Mexican and US officials entered the tunnel from opposite ends
US border officials have unearthed the longest and deepest tunnel ever gouged under the country's Mexican border.
The tunnel begins near the Mexican city of Tijuana, travels under a heavily fortified stretch of frontier, and is equipped with lights and ventilation.
Hacked out of the earth some 26m (85ft) below ground, the tunnel runs 720m (2,400ft) between two industrial warehouses straddling the border.
Officials discovered about two tons of marijuana while exploring the tunnel.
Of more surprise to immigration and customs officials in the US, who discovered the tunnel's entrance, was the route's sophisticated underground infrastructure.
As well as electric lights and ventilation, the tunnel has a concrete floor, groundwater pumping and a pulley system for winching goods in and out.
The tunnel was discovered on Wednesday evening by US agents at a warehouse in the Californian town of Otay.
They alerted Mexican authorities and both sides began exploring the tunnel.
Eventually the source was traced to another warehouse on the outskirts of Tijuana.
"It's just huge, absolutely incredible," said Michael Unzueta, a US immigration and customs special agent based in San Diego, California.
"We believe this tunnel is, in fact, the largest tunnel ever found on the south-west border."
"Our quick assumption is it's the drug cartels," he said, adding that those responsible for constructing the tunnel needed access to money and connections in construction and engineering.
However, the size and scope of the tunnel meant it could have been a conduit for other criminals or terrorists, Mr Unzueta added.
"We're very concerned. When we find these tunnels, we see that as a vulnerability to our national security."
Two shorter tunnels were found earlier this month underneath the border, one ending in California and the other in Arizona.
The US and Mexico share a 3,200-km (2,000-mile) border that is major pathway for drugs and illegal immigrants into the US.