The Sensex at the Bombay Stock Exchange on Wednesday declined 169 points, or 0.9 percent, to close below the 18,000 mark, at an over six-week low of 17,941 on continued selling by investors, amid falling global markets. The Nifty at the National Stock Exchange lost 52 points, or 1 percent, to 5,405. The bourses in Japan, Hong Kong, Singapore and South Korea slumped between 1.5 percent and 2.5 percent, on mounting concerns about slowing global economic growth, and shaky sovereign debt.