The benchmark Sensex dropped 558 points in opening trade. The Sensex fell to as low as 16,432.00 points, its lowest level since June 1 last year, with a fall of 558.18 points from Monday's close. Another benchmark index, the Nifty, slipped below the 5,000-mark for the first time since June 10, 2010, in opening trade. It was trading 172.05 points down within minutes of the opening bell at 4946.45, its lowest level since May 27 last year.