The Sensex at the Bombay Stock Exchange plunged 340 points, or 1.9 percent, to close below the 18,000-mark, at an over 11- month low of 17,906, today, as the rupee dropped to a new all-time low. The Sensex has now lost over 1,460 points in the past four trading sessions. 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.