Tracking falling global markets, the Sensex at the Bombay Stock Exchange dropped 316 points, or 1.8 percent, to close below the 17,000 mark, at 16,990, today. The Nifty at the National Stock Exchange lost 93 points, or 1.8 percent, to 5,119. The market fell on sustained selling by investors, following the downgrade of US government debt by global rating agency, Standard & Poor's.