The S&P BSE Sensex plunged over 550 points in trade following Asian markets, which were trading lower as investors are starting to worry that the prolonged slump in oil prices is signalling a weaker global economy. Tracking the momentum, the 50-share Nifty index also cracked over 181 points to slip below its crucial psychological level of 8200, weighed down by losses in auto, banks, capital goods and IT stocks.