Drug kingpins smuggle vast amounts of cocaine and other drugs into the United States, where they sell it in bulk. They don't take credit cards or checks, only cash will do.
That amounts to a lot of greenbacks that traffickers then want to get to Mexico and other points south.
Decades of efforts to seize drugs entering the United States and to wipe out drug production in source countries like Colombia have not reduced the availability of drugs on the streets of America. So U.S. agents are now also focusing on drug traffickers' revenue.
"The drugs don't seem to dissipate the more we seize, so we're looking at trying to cut off their lifeline to the money, because that money is going to finance the next cycle of drugs that come into the United States," Don Semesky, head of financial operations of the U.S. Drug Enforcement Administration, told The Associated Press.
Traffickers smuggle $6 billion to $22 billion in cash into Mexico from the United States each year, Semesky said in a telephone interview from DEA headquarters near Washington.
Moving that amount of cash, destined mostly for Mexican and Colombian drug cartels, is no easy feat.
A million dollars in $20 bills, often the smuggled denomination equals 50,000 bills weighing 117 pounds. In a single stack it would reach 16 feet, 8 inches high. One million bucks in $100 bills weighs over 23 pounds and would reach 3 feet, four inches high.
Six billion dollars, the minimum estimated total smuggled each year, amounts to 351 tons in $20 bills. In $100 notes, $6 billion weighs 70 tons.
To transport the cash, traffickers often use the same smuggling methods and routes that brought the drugs into the United States.
The drugs go north across the U.S.-Mexican border. The cash heads south.
U.S. drug agents have found bulk cash packed into car doors, secreted in tires or simply buried under a load of bananas and other produce or merchandise in trucks.
Smuggling of bulk cash along the U.S.-Mexican border "is rampant," said Jere Miles, a representative of the U.S. Immigration and Customs Enforcement at the U.S. Embassy in Mexico City.
On Oct. 13, for example, Immigration and Customs Enforcement agents seized more than $2 million near the border in Santa Rosa, Texas, and arrested a Texan who was allegedly carrying the cash in two suitcases in his tractor-trailer.
And that was just small change.