American authorities have been left stunned after discovering the longest drug-smuggling tunnel ever built along the US-Mexico border.

Right under US President Donald Trump’s nose, criminals have been transporting illegal substances across state lines, showing the limitations of his plan for a border wall.

Stretching 1.3 kilometres, the tunnel, which runs from an industrial site in the Mexican town of Tijuana into the US city of San Diego, is considered to be the length of more than 14 US football fields.

It also runs far below the current underground limit of Mr Trump’s border wall.

Decked out with a rail cart system, lift, high-voltage electrical cables, ventilation and a drainage system, the tunnel is 21 metres below the surface of the ground in an area that is a stronghold of Mexico’s most powerful drug syndicate, the Sinaloa cartel.

“This one blows past [the previous tunnel record],” Border Patrol operations supervisor Lance LeNoir said.

The next longest tunnel in the US, discovered in San Diego in 2014, was just over 904 metres.

“We never really thought they (drug cartels) had the moxie to go that far,” Mr LeNoir said.

“They continue to surprise me.”

Authorities discovered the “high-level narco-tunnel” in August 2019. Photo: US Customs and Border Protection

Authorities are yet to make arrests or seize drugs and have not revealed who they think is responsible for the sophisticated tunnel.

Their priority is to seal off the US side of the tunnel by filling it with concrete, US Drug Enforcement Agency leading agent John Callery said.

“The sophistication of this tunnel demonstrates the determination and monetary resources of the cartels.”

While cross-border tunnels at the US-Mexico border are nothing new, the tunnel’s size and its technical components shocked authorities.

The tunnel was discovered in late August 2019 by the US Customs and Border Protection and Mexican law enforcement.

Deputy Chief Patrol Agent Aaron Heitke said he was “thrilled” by the discovery of the “high-level narco-tunnel” – and it would be “rendered unusable for cross-border smuggling”.

An offshoot from the main tunnel was discovered at about one kilometre into the US. It travelled several metres before coming to an end without breaching the surface.

The main tunnel extended another city block at which point agents discovered several hundred sandbags blocking its suspected former exit at a warehouse district in San Diego.

-with agencies