At 905 feet, the tunnel discovered in Naco, Ariz., this week is almost twice the length of the previous record holder.

When a U-Haul truck carrying 4,700 pounds of marijuana rolled into Bisbee, Ariz., this week, the local cops were ready. The local police had been tipped off and, with the help of the Border Patrol’s canine team, managed to intercept the truck and its $3 million-worth of bundled cargo. The operation was a success, but that was just the beginning.

Tracing the U-Haul’s route led the cops and U.S. Border Patrol agents to a house in the border town of Naco, where, inside a small shed on the property, they found the entrance to the longest drug-smuggling tunnel ever discovered in the Tucson sector.

According to a press release from U.S. Immigration and Customs Enforcement (ICE) and Customs and Border Protection (CBP), both agencies of the Department of Homeland Security, specially trained agents from the Border Patrol’s tunnel team were immediately dispatched to begin investigating the underground passageway. Despite an initial delay, due to concerns over the tunnel’s air quality, the investigators found a hydraulic lift inside a cement shaft, leading down into the wood-bolstered tunnel. At least near the entrance, the tunnel is big enough to allow an adult to stand up straight.

This video offers a glimpse of what the tunnel looks like from the inside.

Tunnels have long served as a popular means of transporting drugs between Mexico and the United States, especially as recent increases in U.S. agents and fencing along the Southwest border have forced cartels to come up with alternative points of entry. The first cross-border smuggling tunnel was discovered by the Border Patrol in 1990. Since then, a total of 168 tunnels have been identified and, since 2006, 80 have been found and dismantled.

CBP Arizona tweet:

Did you know, the tunnel discovered in Naco on Monday is 905 ft. long and the longest ever discovered in the Border Patrol’s Tucson Sector? — CBPArizona (@CBPArizona) February 26, 2015

At 905 feet, the tunnel discovered in Naco this week is almost twice the length of the previous record holder, a 481-foot underpass discovered last year in Nogales, Ariz., considered the most active border area for underground tunneling. And while officials described the smuggling shaft as “sophisticated,” these Tucson tunnels hardly compare to the wide-ranging underground transportation system authorities have discovered beneath San Diego in recent years.

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Abundant warehouses and easy-to-dig soil in both San Diego and Tijuana have made the space between the two cities a cartel favorite for underground smuggling. Ranging in length up to 2,400 feet, some of the state-of-the-art tunnels tapped in this area in recent years include such features as electricity, ventilation, groundwater pumping, pulley systems and even a multitiered electric railway.

A few of these sophisticated tunnels have been traced back to the Sinaloa cartel, one of the world’s largest and most notorious drug-smuggling rings, including one discovered in 2012 and another that was shut down by U.S. authorities in November 2014. In fact, after his arrest in 2014, it was revealed that Joaquín Guzmán Loera, the elusive head of the Sinaloa cartel, and better known as “El Chapo,” had relied on a network of underground tunnels to evade authorities.

In the cat-and-mouse game between Border Patrol agents and drug smugglers, beefed-up security sends smugglers underground, and Border Patrol develops new technology to go in after them. In addition to establishing tunnel task forces of specially trained agents along the Southwest border, the Border Patrol also employs robots with cameras strapped to them to explore some of the narrower, potentially dangerous passageways.

“With this discovery, we’ve dealt another significant blow to the Mexican drug cartels and proven yet again that going underground is not a workaround to evade law enforcement,” Matthew Allen, special agent in charge of ICE’s Homeland Security Investigations in Arizona, said in a statement about the Naco tunnel.

So far, two men, the truck driver and another suspect found at the house in Naco, have been arrested. The investigation is ongoing, with Mexican law enforcement now searching for the opening on their side of the border. If previous passageways are any indication, this tunnel could lead investigators to far more people than just those two.