The 30-scrip sensitive index (Sensex) of the S&P Bombay Stock Exchange (BSE), which opened at 18,460.72 points, traded at 17,948.43 points around 3.00 p.m., down 609.70 points or 3.29 percent from the previous day's close at 18,558.13 points. This was the second time the Sensex traded below the 18,000-mark.