The Sensex at the Bombay Stock Exchange opened 72 points, or 0.4 per cent lower, at 17,798, this morning, on sustained selling by foreign funds and retail investors, amid subdued Asian markets. Later, the Sensex extended its losses, and had lost a massive 556 points at one stage. After recovering somewhat, the Sensex still stood a good 249 points, or 1.4 percent in negative territory, at 17,622 in afternoon deals, a short while ago. The 30-share Sensex has already lost almost 440 points in the past three trading sessions.Other Asian bourses in Japan, China, Hong Kong, South Korea, Singapore, Taiwan and Indonesia were trading mixed, today, amid concerns about Greece's soverign debt, and global growth prospects.