Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange tumbled a huge 556 points, or 2 percent, in a broad-based sell-off, to close below the 28,000-mark, at an over three-week low of 27,886. The Sensex has now lost almost 1,160 points in the last four sessions. The Nifty at the National Stock Exchange plummeted 158 points, to 8,448. And the rupee posted its biggest single-day fall so far this year, plunging 55 paise, to an over one-month closing low of 62.91 against the dollar, on a falling equity market, and dollar demand from importers.