The Sensex at the Bombay Stock Exchange dropped 90 points, or 0.5 per cent, to 18,768, in opening trade, on Monday morning, on continued selling by investors, amid weak cues from the global markets. Later, the Sensex lost further ground, and stood 137 points, or 0.7 percent in the negative zone, at 18,721, in afternoon deals, a short while ago.Stock markets in Japan, Hong Kong, Indonesia, Singapore, South Korea and Taiwan were down by between 0.3 percent and 1 percent, today, after poor employment data in the US dimmed the outlook for global economic recovery and Asian exporters' earnings.