Snapping four days of gains, the Sensex at the Bombay Stock Exchange plummeted 651 points, or 3.5 percent, to close at 18,235, today, amid rupee weakness. The Sensex had gained over 900 points in the previous four trading sessions. The Nifty tumbled 209 points, to 5,341. But stock markets in Japan, China, Hong Kong and South Korea gained between 0.5 percent and 3 percent.