'Nightline' Granted Exclusive Access to U.S. Drug Enforcement Officials, Learns Mexican Drug Cartels Trying to Expand Marijuana Shipments to the U.S. Exclusive Access to DEA Agents Uncovers Massive MexicanDrug Cartel Shipments A "Nightline" investigation has discovered that Mexican cartels have recently been trying to expand marijuana shipments into the United States by the tens of thousands. "Nightline" was granted exclusive and unprecedented access to U.S. DEA agents and Customs and Border Protection officers who interdict, store and destroy tons of marijuana. Our investigation takes a look at the scale and reach of the Mexican cartels who are fueled by 25.8 million American marijuana users. Government investigators estimate that the cartels have boosted their production by a whopping 59 percent since 2003, leading them to conclude that the Mexican organizations "represent the single greatest drug trafficking threat to the United States," an official said. Officials estimate that the drug cartels' profits are between $18 and $39 billion annually. According to Mexican and U.S. officials (who requested that their names and ranks not be used), marijuana smuggling has contributed to 35,000 deaths along the border in the past five years. These discoveries come on the heels of a bill being introduced in the House by Reps. Barney Frank (D-Mass.) and Ron Paul (R- Texas) to remove the federal roadblock to state marijuana reform. "Nightline" was present in the midst of a two-week stretch of U.S. officials' destroying more than $100 million worth of marijuana at two top-secret facilities in an undisclosed location in the American Southwest. Watch the full report on "Nightline" tonight at 11:35 p.m. ET Our cameras rolled as Customs and Border patrol agents stopped an 18-wheeler carrying furniture. But an X-ray of the pottery and wooden furniture inside the cargo hold showed something else. "We pried open the ottoman and we found nine cylinder-shaped packages containing marijuana," said Supervising Officer Alberto Flores of Customs and Border protection. Officers went to work slicing open more than 100 ottomans that were packed with 900 containers of marijuana. There was about 2,300 pounds of marijuana in this one shipment -- more than a ton. "Marijuana is the No. 1 cash crop for the cartels in Mexico," said assistant special DEA agent Mel Rodriguez. "The moneys, the proceeds from the sale of the marijuana ultimately go to finance other illegal activities for the cartel, such as [the] purchase of weapons and additional resources." "Additional resources" include funding armies of criminals who have fought the U.S. and Mexican governments. U.S. officials use a variety of tools to find contraband, including an army of agents, Border Patrol's drug-sniffing dogs, mobile X-ray machines, even special cameras to slide down gas tanks to hunt for drugs. "We find sometimes narcotics inside batteries, inside carburetors, manifolds, underneath dashes, inside air conditioning compressors, inside tires, in the roof. I just can't think of all the places we have found them," said Port of Laredo acting director Jose Uribe. Drug lords use every tactic to transport drugs, cash and guns: submarines, tunnels, ultra-light planes. They also still use men on foot -- so-called "mules." After U.S. agents seize the drugs, they are moved into a secret facility -- one of the most restricted government rooms in the nation that, until now, no television journalist had ever been allowed in before. We had to sign papers just to walk from room to room, and no employee working there could be identified in our report. U.S. Officials Burn Several Tons of Marijuana Siezed From Mexican Drug Cartels About $59 million worth of marijuana and every other drug you can imagine being captured coming in from Mexico is stored in this secret vault: cocaine, crystal methamphetamine, heroin. An agent pointed to a pack of 31 pounds of black tar heroin. "Estimated street value about $1 million," he said. We were granted a ride-along with a DEA caravan transporting confiscated drugs, with armed guards behind the wheel. Assistant special DEA agent Mel Rodriguez explained that the particular load we were traveling with was 320 pounds of drugs that was passed off to the DEA by Border patrol. "They encountered six individuals who had just crossed over with duffel bags," he explained. "As soon as Border patrol approached them, they wound up dropping the duffel bags and jumping back into the river and swimming back to Mexico." At some point the U.S. government has to destroy the drugs. DEA officers took "Nightline" inside another top-secret location where the agency was in the process of destroying 50,000 pounds, or 25 tons, of marijuana. Just like with the other vault, we were not allowed to disclose its location nor identify who worked there. The vault is well protected with cement and steel. Surveillance cameras are everywhere, not to mention the throngs of armed DEA agents all over the place. Each cache of marijuana must be accounted for and is given a number as it is brought inside. There is so much marijuana in the facility that the drugs have to be moved by a forklift. When it is time to destroy a haul, the marijuana is carried by forklift, then dumped into a large shredder before being fed into two huge incinerators and burned at more than 1,800 degrees. Some of the agents have even nicknamed the incinerators Dorothy and Bong 1. After the marijuana is burned to ash, it is placed into barrels. Only steam from the heat of the incinerators is released into the atmosphere as millions and millions of drugs go up in smoke. 'Nightline' Granted Exclusive Access to U.S. Drug Enforcement Officials, Learns Mexican Drug Cartels Trying to Expand Marijuana Shipments to the U.S.