The workmanship of the Supernote was extraordinary. It had sequential serial numbers, and the printing plates continued to be refined. A Secret Service agent identified Kelly" s two samples as Supernotes by three minuscule imperfections. Even when the flaws were pointed out, Kelly says, " I frankly couldn" t see the damn imperfections. " Most alarming of all, the Supernote was so well engineered that it could fool currency scanners at the nation" s twelve Federal Reserve banks. The black ink on the front of American currency contains ferrous oxide, which is magnetic, and the Fed" s scanners read the magnetic field down the center line of the portrait with such precision that a thousand genuine hundred-dollar bills are rejected for every one that is later found to be counterfeit. Yet, Kelly recalls, " Secret Service told me the bills went through those machines". The Supernote, Kelly learned, had been circulating in Europe, the Far East, the Middle East, and the former Soviet Union, but only limited supply had reached the United States. This was not reassuring. Of the almost three hundred and ninety billion dollars in American paper money now in existence, some two-thirds, or more than two hundred and fifty billion, is in foreign hands. The worldwide popularity of the dollar is a tremendous boon to the United States. The Federal Reserve is fond of pointing out, every bill in circulation is in effect an interest-free loan; an equivalent amount in government securities would cost the United States more than twenty-five billion dollars in annual interest payments. The beauty of bills stuffed in a mattress in Kazakhstan, for instance, is the good chance that the notes will never be called in. The Supernote was by no means the first foreign-made or foreign-distributed counterfeit of American currency, but because of its frightening and unprecedented quality it seemed singularly poised to damage world confidence in the dollar.