Falling in tandem with other global markets, the Sensex at the Bombay Stock Exchange plunged 311 points, or 1.8 percent, to 17,380, today, after credit-rating downgrades of Greece and Portugal fueled concern among investors, worldwide, about sovereign defaults. The Nifty at the National Stock Exchange plunged 93 points, or 1.8 percent, to 5,215.Stock markets in Japan, Hong Kong, Singapore and South Korea slipped between 1 percent and 2.5 percent.