Falling for the second straight session, the Sensex at the Bombay Stock Exchange declined 137 points, or 0.7 percent, to 18,721, on sustained selling by investors, amid weak global markets, today. The Nifty at the National Stock Exchange lost 45 points, or 0.8 percent, to 5,616. Stock markets in Japan, Hong Kong, South Korea, and Singapore lost between 0.7 percent and 1.7 percent.