A task force led by U.S. Immigration and Customs Enforcement's Homeland Security Investigations also arrested three men late Monday and seized more than 640 pounds of marijuana and a half-pound of heroin.
Authorities say the 481-foot tunnel runs between two private residences accessed by shafts at each end.
The tunnel stretches 411 feet from the U.S. entrance to the international border and extends for another 70 feet into Mexico.
The passageway is about 2 feet wide by 3 feet tall and contains wood shoring, electric lighting and fans to circulate air inside.
Two of the arrested men are from Mexico while the other is from Nogales, Ariz.