Stocks plunged at the opening bell Monday, triggering a 15-minute market-wide trading halt as investors failed to take comfort in the Federal Reserve's emergency policy measures. The S&P 500 SPX, -1.11% fell 220.55 points, or 8.1%, at the opening bell, immediately triggering a trading curb that kicks in at a loss of 7% for the benchmark index. The Dow Jones Industrial Average DJIA, -0.87% dropped 2,250 points, or 9.7%, while the Nasdaq Composite COMP, -1.07% droped 482 points, or 6.1%. Another circuit breaker would kick in if the S&P 500 falls 13% once trading resumes.