According to media reports, a trader spent a lot of money buying call options linked to the "panic index" to bet on a significant rebound in volatility in the US stock market over the next month. The CBOE Volatility Index (VIX), also known as the Wall Street Panic Index, reflects the implied volatility of S&P 500 index futures over the next 30 days and serves as a barometer for measuring stock market volatility expectations. The VIX and S&P 500 index usually have a negative correlation, meaning that when the stock market rises, the VIX index often falls, and vice versa.