The 2,400-feet long passageway is longer than most of the 21 cross-border tunnels that have been discovered since authorities began keeping track after the Sept. 11 attacks, U.S. Immigration and Customs Enforcement officials said.
"It was like being in a cavern or a cave," said Michael Unzueta, customs special agent in charge in San Diego.
The tunnel's discovery prompted the U.S. Attorney's office in San Diego to open a criminal investigation, said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement.
Mexican authorities found the entrance about 100 yards south of the border on Tuesday, and officers on the U.S. side found the exit Wednesday. Mexican officials allowed reporters and photographers, including an Associated Press photographer, into the tunnel late Wednesday.
The tunnel was about five feet wide and high enough for an adult to stand inside, had a cement floor, and lights mounted on one of the hard soil walls. It was equipped with a pulley system on the Mexican side.
Four tunnels have been discovered this month in the Tijuana-San Diego area, including a more primitive tunnel that was also found Wednesday when a U.S. Border Patrol vehicle struck a sinkhole.