Great BBC on counterfeit US currency. One theory is that it is act of economic terrorism.
What is a superdollar?
The superdollar is the name given to an almost perfect forgery of an American banknote.
Millions of dollars of the fake cash have been passed into circulation since its existence was first noticed over a decade ago.
The money, officially known as Note Family - C14342, is thought to originate from communist North Korea.
Experts believe that the money is being produced and flooded into the system, mostly by North Korean diplomats as they travel abroad.
It is also circulated by criminals - with the Russian mafia and even Republican organisations in Northern Ireland involved in the distribution process.
I brought some of these counterfeit notes to South Korea, and I showed them to the experts in the South Korean intelligence agency. They said these are not fake notes; they're real
North Korean defector
One school of thought is that it is part of a plan to try to destabilise the American economy by putting millions and millions of dollars into the system.
You can read the rest on the BBC site: What is a superdollar?