Authorities in California have seized a massive amount of drugs and arrested nearly two dozen people after finding the tunnel.

According to officials twelve tons of marijuana were seized during the operation, which shut down one of the longest cross-border tunnels ever discovered. 22 people were arrested after police discovered the passage, which ran between warehouses in San Diego, California, and Tijuana, Mexico, writes Elliott Spagat for The Associated Press.

Huge Drug Tunnel Found At U.S.-Mexico Border

Mexico’s sophisticated tunnel would have shipped tons of marijuana into U.S.

The tunnel measured around 2,400 feet in length and was carved out some 30 feet below the surface. It boasted lights, ventilators and a rail system, which are hallmarks of the most high-tech tunnels used by drug smugglers along the border.

U.S. and Mexican authorities carried out sting operations at almost the same time on Wednesday, arresting 6 people in San Diego and 16 in Tijuana. Officials found 2 tons of marijuana in the U.S. and 10 tons awaiting shipment in Mexico.

According to U.S. authorities, the smugglers attempted to move a load of drugs through the tunnel on Wednesday, but their movements were detected. A probable cause statement reveals that the operation was made possible after an undercover U.S. Homeland Security Investigations agent offered the smugglers the use of a warehouse and drivers, on the condition that they paid him $10,000 for each truckload of drugs shipped.

Cross-border tunnels remain popular with drug smugglers

The operation provides a window into the strategies employed by drug smugglers. Tunnels remain very appealing, despite the fact that they take a lot of time and money to build. In recent years there have been a number of tunnels discovered under the border, the most sophisticated of which have been equipped with hydraulic lifts and electric rail cars.

Due to its clay-like soil the San Diego-Tijuana area has proved popular with smugglers because it is easier to dig with shovels and basic tools than in other regions. In addition there are warehouses on both sides of the border which can be used to conceal the activity of trucks and heavy equipment.

Authorities have not revealed which drug trafficking organization was responsible for the tunnel, however the area is known to be largely under the control of the Sinaloa cartel. The Mexican cartel’s leader, Joaquin “El Chapo” Guzman, used a sophisticated tunnel to escape from his maximum-security jail in July.

According to Mexican federal police, the people arrested at the tunnel said that they were linked to a cartel operating in Jalisco, which may be a reference to the Jalisco New Generation cartel that controls the area.

Sting operations leads to federal and state charges in U.S.

In Mexico the suspects found at the tunnel were surprised when police showed up with a search warrant. No shots were fired during the operation, as police discovered 873 packages of drugs wrapped in plastic and tape.

U.S. Attorney Laura Duffy said that suspects running the tunnel on the U.S. side appeared to have been using pulleys as there were no stairs or ladder into the passage itself. The tunnel emerged into a warehouse near the Otay Mesa border crossing.

David Shaw, head of Homeland Security Investigations in San Diego, told the press that there had been a spike in activity at the San Diego warehouse leading up to the sting. “It almost looked like they filled it with junk to look like they were busy,” Shaw said.

The investigation into the site began in May, he said. Two Tijuana residents, Isaias Enriquez, 53, and Isidro Silva, 27,  have been charged with conspiracy in U.S. federal court. Four others will be charged in a state court.

Successful sting operation shuts down operation

In recorded conversations Enriquez can be heard making a deal to pay $10,000 per truckload to an undercover agent who promised him the use of a second warehouse and drivers, according to the probable cause statement. The second warehouse was to receive shipments from the warehouse on the border in San Diego.

On Wednesday two agents unloaded 249 packages of marijuana at the second warehouse, before one of them met Enriquez and Silva at a restaurant in San Diego to discuss another shipment.

At the beginning of August another tunnel was discovered in Tijuana, but it had not been finished and ended short of the U.S. border. Despite strict security protocols, Mexican cartels are constantly finding new ways of smuggling drugs into the U.S., where ravenous demand drives profit for the gangs.

These profits are then reinvested in ever more sophisticated smuggling techniques, engaging the authorities in a never-ending game of cat and mouse with the cartels. A new approach is needed to break the cycle.