Falling for the second straight day, the Sensex at the Bombay Stock Exchange plunged 363 points, or 1.8 percent, to close below the 20,000 mark, at 19,901, on selling by investors. The Nifty at the National Stock Exchange dropped 122 points, or 2 percent, to 5,890.Stock markets in China, Hong Kong, South Korea and Singapore ended mixed. Stock markets in Japan were closed for a holiday.