As stock markets around the world continue into their third day of plunges, the VIX has only been this high on three prior occasions.

The Cboe's Volatility Index, which measures investors' expectations of future stock price fluctuations and is commonly referred to as the "fear index," usually spikes during stock market crashes. Traders can bet on VIX spikes in order to offset their losses from stock declines.

The VIX closed yesterday at 39.94, its fourth "worst" day since the index was created in 1990. The only three periods where the VIX signalled even greater alarm were: