The Sensex today tanked over 431 points to trade below the 17,000 level in opening trade on major sell-off by funds and retail investors amid slumping global markets. The 30-share index, which had gained 95 points in the previous session, plunged by 431 points, or 2.52 per cent, to 16,687 points. All the sectoral indices, led by metal and realty, were trading in the negative with losses up to 3.77 per cent. The wide-based National Stock Exchange index Nifty dipped by 131 points, or 2.55 per cent, to 5,004 points.