US and Mexican authorities have uncovered the longest smuggling tunnel ever found at the two countries’ border.

The sophisticated “narco-tunnel” stretches over three quarters of a mile and is fitted with “extensive” rail tracks, air vents, high-voltage electricity, a drainage system and a lift.

Mexican authorities found an entrance to the underground passageway in an industrial zone in the city of Tijuana last year.

US border agents mapped the route of the 4,309ft (1.3km) tunnel and discovered a suspected exit, blocked with hundreds of sandbags, in a warehouse district in San Diego.

No drugs were found and no arrests have been made, but the tunnel is believed to have been dug by Mexican cartels.

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“The sophistication of this tunnel demonstrates the determination and monetary resources of the cartels,” said Drug Enforcement Administration special agent John Callery.

Smuggling gangs were being “forced underground” by tighter US border security, he claimed.

But the tunnel exposes the limitations of Donald Trump‘s much-vaunted border wall, which stretches several feet underground in the area and is considered effective against small, crudely built tunnels known as “gopher holes.”

The newly discovered tunnel runs at an average depth of 70ft (21.3m), well below the reaches of the border wall. The passage is about 5.5ft (1.7m) high and 2ft (61cm) wide.

Border patrol chiefs were taken aback by the scale of the tunnel, which is far bigger than the previous longest – a 2,966ft passageway found in San Diego six years ago.

“This one blows past that,” said Lance LeNoir, a Border Patrol operations supervisor. “We never really thought they had the moxie [nerve] to go that far. They continue to surprise me.”

US authorities said an ‘extensive’ rail and cart system had been installed in the tunnel (via REUTERS)

The tunnel passes under several warehouses in San Diego’s Otay Mesa district, where smuggling tunnels have typically ended, and extends into open fields.

An incomplete offshoot from the tunnel at a distance of 3,529ft from its starting point suggested to authorities that smugglers had plugged its initial exit point and were working on another.

Border Patrol spokesman Jeff Stephenson said the suspected previous exit “became unsustainable for whatever reason, so they built a spur”.

“The sophistication and length of this particular tunnel demonstrates the time-consuming efforts transnational criminal organisations will undertake to facilitate cross-border smuggling,” said Cardell Morant, acting special agent in charge of US Immigration and Customs Enforcement’s Homeland Security Investigations unit in San Diego.

Authorities have found 15 sophisticated tunnels on California’s border with Mexico since 2006, with hallmarks including lighting, ventilation, railway tracks and hydraulic lifts.

The tunnels’ exits were mostly in Otay Mesa, an area where clay-like soil is conducive to digging and warehouses provide cover.

On the Mexican side of the border, the latest tunnel’s entrance had been concealed by a small building in car park for cargo trucks next to Tijuana’s airport.

After mapping the route, US authorities drilled a hole on the American side and lowered a camera to determine where to start making their way through.

“It was endless,” Mr LeNoir said.

Authorities have not named any suspects but the area where the tunnel began has been a stronghold of Mexico’s Sinaloa cartel, whose longtime leader Joaquin “El Chapo” Guzman was jailed for life in the US in July.