The 627-foot-long subterranean tunnel was first found by Mexico State Police on Sept 19.
Over half of the tunnel, near Jacumba in California, was later found to be inside the United States.
The tunnel was incomplete, with construction apparently having been abandoned just 15 feet from the surface on the U.S. side.
Mexican law enforcement and the military came across the entrance to the tunnel during an operation in Jacume, Baja California, Mexico.
U.S. Customs and Border Protection (CBP) were then called in, along with the DEA and HSI, to help map the tunnel and see if it exited into the United States.
“Agents reported that there was a solar panel system used to run the electrical, lighting, and ventilation systems that were in the tunnel,” said a statement released by the CBP on Oct 9.
The solar setup also powered two sump-pump systems to bail out any water.
The mapping of the tunnel was completed on Oct.4.
“The tunnel entry point had a shaft that was approximately 31 feet deep, with a total length of 627 feet, of which 336 feet were inside the United States,” the CBP statement read.
The tunnel averaged three feet in height and was roughly 2 1/2 feet wide.
“Agents reached an exit shaft that went approximately 15 feet up toward the surface but did not break the surface and did not have an exit point into the United States,” according to the CBP.
‘A Lot of Time and Money’
The tunnel also included a rail system that ran the entire length of the tunnel.
“Sophisticated tunnels take a lot of time and money to make,” said Border Patrol Agent Tekae Michael, according to the San Diego Union-Tribune. “When we find them, they’re a pretty big deal.”
The tunnel was likely made to transport drugs into the U.S. from Mexico, Michael said.
No arrests have been made on the U.S. side, but the tunnel is still under investigation, Michael said.
Just over 230 tunnels have been found on the U.S.-Mexico border since 1990, according to the latest annual report from the DEA.
Tons of Marijuana Smuggled
Nearly 200 of these actually crossed into the United States territory, though the majority of them do not reach the surface on the U.S. side.
“Underground tunnels are mainly used to smuggle ton quantities of marijuana, though there are instances of other illicit drugs commingled in shipments,” according to the 2017 National Drug Threat Assessment.
Smuggling drugs through border checks, concealed on pedestrians and in cars is the preferred smuggling method for methamphetamine and opiates.
But the giveaway odor of marijuana is harder to mask, and the drug is far bulkier, so cartels resort to the tunnel method.
The tunnels have been primarily found in California and Arizona, and are generally associated with the Sinaloa Cartel.
The Sinaloa Cartel dominates drug smuggling in the area of Mexico near San Diego, its relatively vast resources and money allowing for the construction of more sophisticated tunnels.