The Sensex at the Bombay Stock Exchange declined a massive 406 points, or 2.2 percent, to slip below the psychological 18,000 mark, and stand at 17,773, in afternoon trade, a short while ago. Earlier, in the morning, the Sensex had opened 90 points lower, at 18,088. The markets fell on sustained selling by investors, with sentiment dampened by rising crude oil prices, amid the political unrest in Libya, and weak global bourses.Key Asian markets in Japan, Hong Kong, South Korea, Singapore and Taiwan were down by between 0.4 percent and 1.5 percent, on continuing concern about the political instability in West Asia.