U.S. markets are opening this morning after their worst day in almost three years. The Dow Jones index of thirty blue-chip stocks closed more than 500 points down, or 4.3 percent, the biggest one-day fall since late 2008. Indexes including the S&P 500 also plunged yesterday.  The news reflects fears of a slowdown in global growth, concerns about Europe’s debt crisis, and the prospect of a double dip recession. Louise Story, Wall Street and finance reporter for our partner,  The New York Times, explains why the market plunge occurred.