The Sensex at the Bombay Stock Exchange plunged 300 points, or 1.6 percent, to close at 18,243, dragged down by IT stocks, particularly Infosys, and as regional and European markets lost ground, today. The Nifty at the National Stock Exchange dropped 65 points, or 1.2 percent, to 5,529.Stock markets in Japan, China, Hong Kong, South Korea and Singapore declined between 0.1 percent and 1.3 percent.