The tunnel was not large enough to allow a person to stand up, but it was equipped with electricity, ventilation and even a rail system used to move marijuana into the U.S. in carts.
Federal agents made the drug bust after a month-long investigation working with Mexican authorities.
In all, officials on both sides of the border confiscated more than $20 million worth of marijuana.
"It is a very significant discovery because of the amount of narcotics that were seized," said John Morton, director of the /*U.S. Immigration and Customs Enforcement*/.
Agents spent hours staking out the area surrounding the tunnel last night, then took down the driver of a big rig and his wife after they left a warehouse near the tunnel.
The truck was carrying about 10 tons of marijuana.
With a search warrant in hand, agents were led from the warehouse to the opening of the tunnel, which ran the length of six football fields.
"We have disrupted a major tunnel, a major drug trafficking organization," Morton said. "This wasn't, at 30 tons, a mom and pop organization. This is obviously the work of the cartels."
Officials have found 125 cross-border tunnels built by Mexican drug cartels since the early 1990s. The majority were found along the California-Arizona border with Mexico.