The Sensex at the Bombay Stock Exchange fell 146 points, or 0.9 percent, to 16, 736 in opening trade, this morning, on sustained selling by investors, amid considerable weakness in the regional markets.Later, the Sensex lost some more ground, and stood 192 points or 1.1 percent in negative territory, at 16,690 in afternoon deals, a short while ago.The Sensex has already lost more than 680 points in the past four trading sessions.Other Asian bourses in Japan, China, Hong Kong, Indonesia, South Korea, Singapore, and Taiwan were down by between 0.9 percent and 2.7 percent, today, on fears that even core Euro zone members may not escape contagion from the region's two-year old debt crisis.