The Bombay Stock Exchange Sensex plummeted by over 223 points in opening trade today to trade below the 18,000-mark on continued selling by foreign funds and retail investors, spooked by the continued uncertainty in global markets.The 30-share barometer, which lost 204 points in yesterday's trade, slid further by 223 points, at 17,887 in the first few minutes of trade today.Similarly, the wide-based National Stock Exchange Nifty moved down by 71 points, to 5,386.