US authorities have announced the discovery of what they said was the longest smuggling tunnel ever discovered on the south-west border, a passageway stretching three-quarters of a mile from Tijuana to a warehouse district located 15 miles south of San Diego.

US Customs and Border Protection said on Wednesday that the tunnel runs more than 4,300ft (1,313 meters), at about 70ft below the surface.

The sophisticated passageway includes an extensive rail-cart system, a ventilation system, high voltage electrical cables and panels, a drainage system and an elevator. Inside, the tunnel reaches 5.5ft high. Agents also spotted several hundred sandbags disguising the entrance to what they believe was the tunnel’s previous exit

The discovery did not lead to any arrests or seizures.

It was far from the first of its kind. The first cross-border smuggling tunnel was discovered in 1990, a project of drug kingpin Joaquín “El Chapo” Guzmán, then-head of the Sinaloa cartel, as he looked for new ways to carry drugs to the US undetected. Since then, authorities have unearthed scores of tunnels along America’s southern border, from Texas to California.

Cardell T Morant, acting special agent in charge of Homeland Security Investigations in San Diego, said that the length and features of the newly discovered tunnel testify to the time and resources smugglers invested in its creation.

Deputy Chief Heitke added, “The investigation continues, and I am confident that our hard work and dedication to uphold the law will lead to future arrests and seizures.”

The tunnel was discovered in August, and after Mexican law enforcement identified the entrance, a tunnel task force in San Diego mapped its route and released their findings on Wednesday.

Over the years, the task force has turned to ground-penetrating radar devices to help sniff out tunnels, but the rocky soil and web of interconnecting sewage systems that run through the San Diego area interfere with the technology.

Tunnels are just one method of moving drugs or people into the US. Migrants in the area are known to cut through the border-fence and dash toward nearby warehouses, where they hide and wait for a driver to later carry them north. Along the coast, just a short trip west, smugglers or migrants will try to land boats on the shores of the US.

The area near the border along San Diego is heavily fortified and militarized. Border agents routinely patrol the area, looking for people trying to scale or cut through the fence.

Despite Donald Trump’s continued focus on building a wall, an estimated 80% to 90% of narcotics enter the country by way of vehicles passing through the US ports of entry.