A surprising finding from New York Times: Economists once believed that just 10 percent of all currency in circulation was tied up in the underground economy. Now it turns out that 80 percent is closer to the mark. Most of it is in $100 bills, which are the largest available and now account for well over half the value of the nation's paper money. American banks and businesses hold very little cash. American households are also loath to keep much of it lying around. Yet according to the Federal Reserve there is enough currency floating around to make it appear that every American family of four has $6,000 or so in cash stashed under the mattress, including three dozen $100 bills. And despite the ubiquitousness of credit cards, ATMs and electronic transfers, the sea of cash has been growing much faster than the nation's gross national product. - Igor.