Mexican drug cartels have achieved a remarkable feat of illicit civil engineering with the construction of two underground rail tunnels connecting Tijuana to San Diego.

US federal agents have discovered a pair of sophisticated underground rail tunnels traversing the US-Mexico border, built by Mexican drug cartels to smuggle their illicit wares to the California city of San Diego.

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The agents report that both of the tunnels connected drugs delivery points in Tijuana, Mexico to warehouses just across the border in the south of San Diego, and were equipped with rail systems to facilitate the process of transportation and delivery.

The terminus point of one of the tunnels was situated in an industrial warehouse stocked with television sets and children’s toys in the San Diego community of Otay Mesa. A 20-metre deep underground shaft fitted with a pulley system was used to retrieve the contraband substances.

The tunnel covered a distance of nearly 550 metres, and was equipped with a crude railway line as well as a lighting system.

The second tunnel covered a distance of over 640 metres and displayed a far more sophisticated level of engineering. The tunnel contained a multi-tier electric rail system, as well as ventilation equipment.

The tunnels are the sixth and seventh cross-border underground tunnels which have been uncovered in San Diego in less than four years.

Federal authorities have uncovered over 80 cross-border smuggling tunnels since 2006, most of which emerge in the border states of California and Arizona.

The Nogales Tunnel Task Force, an initiative of the US Drug Enforcement Agency and the Department of Homeland Security in the Arizona city of Nogales , said that underground smuggling systems have become increasingly sophisticated in the years since the cross-border narcotics trade first began to flourish.

Alex Garcia, a special agent of Homeland Security Investigations in charge of the task force, said the builders of the underground smuggling routes often lacked formal engineering training, yet had managed to obtain extensive knowledge and expertise in the field of tunnel construction following many years of experience.