The Sensex at the Bombay Stock Exchange plunged 322 points to close below the crucial 19,000 level for the first time in four months, at 18,860, today.Sensex closed in red as investors continued to fret that the Reserve Bank will hike key interest rates to tame inflation.The Nifty at the National Stock Exchange also fell 97 points to 5,655 points. Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.