Stock-market trading reopened after a 15-minute halt triggered by a steep slide at the opening bell on Monday. The S&P 500 SPX, -1.11% was down 10.9% near 2,415. Another 15-minute market-wide trading halt will be implemented if the index falls 13% on the day. The initial circuit breaker was triggered when the S&P 500 fell more than 7% at the bell. The Dow Jones Industrial Average DJIA, -0.87% was down 11.8% or 2,700 points, while the Nasdaq Composite COMP, -1.07% dropped nearly 900 points or 11.3%.