At least 12 tons of marijuana were seized and and 22 people detained after a secret "super tunnel" -- eight football fields in length -- was discovered along the U.S-Mexico border south of San Diego.

More than 30 agents from Homeland Security Investigations' (HSI) Special Response Team raided the tunnel Wednesday evening on Otay Center Drive, approximately a half mile away from the Otay Mesa Port of Entry.

As they entered the front room of the Otay Center Warehouse, agents found a 3-foot-wide hole in the floor, which led them to a 32-foot shaft burrowed into the ground.

Images: Border Tunnel Uncovered, Tons of Drugs Seized

The shaft was the entrance to the "super tunnel," as the U.S. Attorney's office called it. The tunnel is believed to have air ventilation, rails to transport drug and a lighting system.

At the bottom of the entrance, agents saw marijuana bundles wrapped in plastic and stacked.

The newly completed tunnel was discovered during a six-month, undercover investigation by the Tunnel Task Force, a joint effort between HSI, the Drug Enforcement Agency and U.S. Border Patrol.

In May, an undercover agent was introduced to a suspected leader of the operation, according to the U.S. Attorney's office. The agent helped transport and store drugs and took buckets of dirt away from the tunnel warehouse, a court complaint details.

#NBC7 confirms Mexican authorities seized 10 tons of marijuana & 16 suspects in connection to #OtayMesa drug tunnel pic.twitter.com/hf2Qnnozxc — Liberty Zabala FOX 5 (@Liberty_Zabala) October 22, 2015

U.S. officials decided to move on the tunnel when the undercover agent met two of the lead suspects at a San Diego restaurant Wednesday. There, they discussed moving the drugs in the tunnel to another warehouse, according to the complaint, so officials believed the load would be going from Mexico to the U.S.

That would have marked the first time the tunnel would be used to move a significant amount of drugs, the U.S. Attorney's office says.

Instead, the Tunnel Task Force shut it down. During the raid, Mexico officials reported 16 arrests and the seizure of 10 tons of marijuana at the Tijuana tunnel entrance. San Diego County Sheriff's deputies arrested four more people in connection with the investigation.

Federal investigators seized two additional tons of marijuana and arrested another two men -- Isaias Enriquez-Acosta and Isidro Silva-Acosta -- on charges of unlawful conspiracy to import a controlled substance and conspiracy to use border tunnels and passages.

The other detainees were aged between 21 and 50 years old.

Officials are still pulling drugs from the tunnel, so the amount seized could rise. As of 3 p.m. Thursday, the discovered marijuana has an estimated street value of about $6 million.

The owner of a business next to the warehouse said he never saw customers walking in or out of what he described as a carpet shop.

He has had his business at the location for more than 20 years and the carpet shop opened up about two months ago.

This super tunnel is the tenth large-scale drug tunnel discovered in the San Diego area since 2006, according to the U.S. Attorney. Federal officials have found more than 75 smuggling tunnels in the last five years, most located in California and Arizona.

Missing Attachment Video from Newschopper 7 shows the tunnel starting point on the Mexico side and then shows images taken inside the tunnel and adjoining businesses.