The tunnel is over 480 feet long and is considered the largest and most sophisticated ever discovered.
It was three feet by two feet, supported by wood, and equipped with electric lights and fans for ventilation.
The giant passage begins inside a home in Nogales, Sonora, Mexico and ends in a home in Nogales, Arizona. Border Patrol and other Federal agents say marijuana, and other drugs have been easily passed into the US for years.
Initial raids found heroin and 46 pounds of pot inside, but another 590 pounds of marijuana was captured from a car trying to leave the house in Arizona.
Three people were arrested including two Mexican citizens in the raid. Nogales has long been a drug smuggling hotbed with nearly 100 tunnels found there since 1990.
Eric Balliet is with ICE's Homeland Security Investigations in Nogales. He told USA Today "if a drug-trafficking organization can establish a well-manufactured tunnel, the likelihood of a law-enforcement encounter is somewhat slim."
"You don't run the risk of exposing your dope aboveground, whether it's human backpackers or smuggling through the port or over the fence. It's completely concealed from start to finish," Balliet continued.
The Feds say Nogales is an easy place for smugglers to use because it has a huge underground drainage system connecting the cities.