Hidden 70 feet beneath a small industrial building in Tijuana, Mexico, lays a long and sophisticated passageway — complete with a railway, plumbing and ventilation system. It stretches the length of almost 12 football fields, and U.S. Customs and Border Protection officials said Mexican narcotics traffickers had used the tunnel to smuggle drugs across the border, into an industrial area of San Diego.

CBP officials announced the discovery of the passageway Wednesday and called it the “longest ever” smuggling tunnel discovered at the U.S.-Mexico border.

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

Smuggling tunnels are commonly used by transnational drug trafficking outfits, though CBP officials said it was still unclear when the tunnel was last used and by whom. Tunnels became closely associated with the Sinaloa drug cartel, partly because of its former leader, Joaquín “El Chapo” Guzmán, who twice used them to escape prison in Mexico. CBP officials said no arrests have been made in connection to the recent tunnel discovery.

The CBP’s Tunnel Task Force initially discovered the passageway in Tijuana, just west of the Otay Mesa Port of Entry, in late August; U.S. officials alerted Mexican authorities, who allowed them to enter the tunnel from Mexico and map its route from the southern side, Theron Francisco, a spokesman with the San Diego Sector of CBP, told The Washington Post.

The mapping process typically takes months. The U.S. exit point was eventually identified in an industrial area of San Diego, and it has been blocked off from the inside by sandbags. The tunnel also had an unfinished dead-end offshoot stretching more than 3,500 feet past the border.

At a total of 4,309 feet in length, the tunnel is one-and-a-half times longer than the previous record-holder identified by CBP officials: a tunnel discovered in San Diego in 2014 that stretched 2,966 feet.

Measuring 5½-feet tall and about 2 feet wide, the Tijuana-to-San Diego tunnel is roomier than most — and better equipped, said Francisco, the CBP spokesman.

“When we classify the tunnels as a sophisticated tunnel, they usually have electricity, a rail system, ventilation. This one had a pretty sophisticated plumbing system for the groundwater runoff,” Francisco said. “When you go 70 feet below and 4,000 feet long, that’s pretty sophisticated.”

Video released by CBP shows standing water on the tunnel floor, with partially submerged hard hats and construction materials bobbing above the surface. Cartels typically dig tunnels using handheld power tools like hammer drills and roto-hammers. Francisco estimated this one would have taken at least a year if not more to complete.

While tunnels beneath the border aren’t new — more than 200 cross-border tunnels have been discovered since CBP was formed in 2003 — the agency expressed hope that exposing and closing the one discovered in August will deal a significant blow to crime organizations that use it for smuggling. Tunnels are occasionally used to bring people into the United States, but Francisco said their primary function is to move drugs — in particular heroin, fentanyl and methamphetamine.

“Drugs are worth more than people, and drugs don’t talk,” he said. “If a person was smuggled through a tunnel, word could get out and that tunnel could be discovered, and [the crime organization’s] investment could be lost.”

As the investigation continues, the tunnel will eventually be closed through what Francisco described as a “pricey” process of drilling holes from the surface, down 70 feet to the tunnel, and then filling it in with a concrete slurry.

“We usually start at the border,” he said. “Ideally, we like to fill the whole thing, but sometimes only portions are filled. Unfortunately, we can’t force Mexico to remediate it.”

President Trump made security at the U.S. southern border a signature promise of his 2016 campaign, exciting his supporters with vows to build an extensive border wall. Roughly $18.4 billion in federal funding has been allocated so far for border fencing during Trump’s administration.

But even as the president has insisted a physical barrier at the border is “the only solution” for what he has labeled a humanitarian and security crisis, smugglers continue to breach the border, both underground and on the surface. As recently as November, border officials acknowledged smugglers were cutting through border fencing with power tools.

In a statement, DEA Special Agent in Charge John W. Callery said administration and its partners were undeterred by the determination and significant financial resources of drug cartels, as evidenced by their expansive tunnels.

“Although the cartels will continue to use their resources to try and breach our border, the DEA and our partners on the Tunnel Task Force will continue to use our resources to ensure they fail, that our border is secure, and that tunnels like this are shut down to stem the flow of deadly drugs entering the United States.”

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