Extending its losses for the eight straight session, the Sensex at the Bombay Stock Exchange tumbled 425 points, or 2.6 percent, to close below the psychological 16,000 mark, at 15,946 on Monday, amid falling Asian and European markets.The Sensex had already nearly 1,200 points in the previous seven trading sessions. The Nifty at the National Stock Exchange slumped 127 points, or 2.6 percent, to 4,778.