The BSE benchmark Sensex plummeted by 260 points in the noon trade today. The 30-share barometer, which was down by 220.70 points, or 1.37 per cent, within minutes of the opening trade was further down by nearly 39 pts or 1.62 percent to trade at 15,805 during noon.The wide-based National stock Exchange Nifty index also fell by 75 points, or 1.57 per cent, to 4,737.