In a dimly lit underground vault a block from Camden Yards, the Federal Reserve is holding millions of dollars in cash that nobody wants... But a 2005 law requires the reserve bank to keep ordering coins regardless of its stockpile, and so vaults in Baltimore and around the country are filling up. ... a 2008 Harris poll that showed 76 percent of Americans prefer paper money. It costs 30 cents to make a $1 coin, but the Fed purchases it for face value - and the U.S. Treasury pockets the difference. In 2010, the Mint put about 400 million $1...