The Sensex at the Bombay Stock Exchange surged 405 points, or 2.3 percent, to close well above the 18,000-mark, at 18,401, yesterday, as the rupee strengthened against the dollar, and as global markets gained ground. The Nifty at the National Stock Exchange spurted 124 points, or 2.4 percent, to 5,409.Stock markets in Japan, Hong Kong, South Korea and Singapore climbed between 0.8 percent and 1.2 percent.