The Sensex at the Bombay Stock Exchange plunged 340 points, or 1.9 percent, to close below the 18,000-mark after nearly one year, at 17,906, today, as the rupee dropped to a new low, amid concerns that the US Federal Reserve will soon rollback its economic stimulus. The Nifty at the National Stock Exchange declined 99 points, or 1.8 percent, to 5,303.Stock markets in Hong Kong, South Korea and Singapore declined between 0.6 percent and 1.1 percent.