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.