As selling on Wall Street intensified, the implied volatility on the S&P 500 as measured by the CBOE Vix index jumped 25% to 18.8, the level last seen in February. The index, known as Wall Street's fear gauge, has been rising since mid-June, when it fell below 11 mark, the lowest levels since 2007. The index is inversely correlated with the S&P 500 and many view it as an indicator of market peaks.
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