Falling in tandem with tumbling global markets, the Sensex at the Bombay Stock Exchange posted its biggest single-day loss in over 26 months, plunging 704 points, or 4.1 percent, to 16,361, on Thursday. The Nifty at the National Stock Exchange plummeted 210 points, or 4.1 percent, to 4,924.Stock markets across the world were battered, after the US central bank disappointed investors with its stimulus plan, and warned of serious downside risks to US economic growth. So the bourses in Japan, China, Hong Kong, South Korea and Singapore sank between 2 percent and 4.9 percent, and the European markets were also hammered down.