The US authorities have discovered 20 tonnes of marijuana, worth tens of millions of dollars, in one of the most advanced illegal tunnels ever found.

The passage is half a mile long and runs from inside a house in Mexico straight under the border with the United States and into a warehouse in San Diego.

The tunnel had its own lighting, ventilation and a system of tracks and trolleys for transporting drugs.

The BBC's Rajesh Mirchandani has been shown around.