It starts in a small industrial building in Tijuana, Mexico, according to United States officials.

From there, the extensive tunnel snakes underground beneath the U.S.-Mexico border, finally ending roughly 4,309 feet later in Southern California, U.S. Customs and Border Protection officials said in a news release on Wednesday.

Federal officials called it “the longest illicit cross-border tunnel ever discovered along the Southwest border.”

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Stretching nearly a mile, this tunnel is more than 1,000 feet longer than the next longest tunnel discovered in the area — a passageway 2,966 feet long that was found in San Diego in 2014, officials said.

The tunnel, which originates about half a mile west of the Otay Mesa Port of Entry, was found in late August 2019 — and after that, Mexican authorities worked with the San Diego Tunnel Task Force to map the sprawling passageway, relying on technology, community outreach and intelligence gathering.

What’s believed to be a former exit to the tunnel was found in San Diego County’s Otay Mesa warehouse district, but hundreds of sandbags blocked access to the tunnel at the location, officials said.

There was also an offshoot to the main shaft that was found about 3,529 feet into the U.S., though that segment of the tunnel abruptly ended without reaching the surface, officials said.

Considerable infrastructure was found inside the tunnel, officials said. It included an “extensive” system of rails and carts, forced ventilation, an elevator at the entry point, high-voltage electrical panels and cables and a complicated system for drainage.

On average, the tunnel runs 70 feet below the ground’s surface and is roughly 5 1/2 feet tall and 2 feet wide, officials said.

“The sophistication and length of this particular tunnel demonstrates the time-consuming efforts transnational criminal organizations will undertake to facilitate cross-border smuggling,” Cardell T. Morant, acting special agent in charge of Homeland Security Investigations in San Diego, said in a statement.

Videos and photos released by federal officials show the dark, dank interior of the gigantic tunnel, including the rail lines “designed to move large packs of drugs.” Other photos reveal tubes for ventilation that were left behind by those digging the tunnel, according to CBP. Agents who explored the subterranean area needed oxygen canisters.

“The tunnel, because it sits under the water table, is flooded in many areas,” CBP officials noted along with one photo showing a water-filled section. “A pumping system was installed by tunnel builders.”

Officials also said that “due to the composition of the underground soil in this area, the tunnel is mostly self-sustaining.”

Deputy Chief Patrol Agent Aaron M. Heitke said in a statement that he’s “thrilled that this high level narco-tunnel has been discovered and will be rendered unusable for cross-border smuggling.”

Still, no one has been arrested and no drugs have been seized as a result of the tunnel being found, according to federal officials.

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