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, on intense across-the-board selling pressure, and rupee weakness. The Sensex had gained over 900 points in the previous four trading sessions. The Nifty at the National Stock Exchange tumbled 209 points, or 3.8 percent, to 5,341.But stock markets in Japan, China, Hong Kong and South Korea gained between 0.5 percent and 3 percent.