Following Friday's mind-numbing 530-point sell-off in the Dow, which sent the blue-chip index into a correction, US stock market futures are plummeting again.

Dow futures were down as much as 850 points. S&P futures are down 101 points. Nasdaq futures are down 208.

At current prices, the S&P 500 has joined the Dow in a correction, which is defined as a 10% decline from most recent highs.

And the sell-off is across asset classes, worldwide.

China's Shanghai Composite crashed 8.5% in what is now being dubbed China's "Black Monday."

European stock markets are getting obliterated, with Germany's DAX falling into a full-blown bear market.

Every stock in Britain's FTSE 100 is in the red.

The CBOE Volatility Index, or the "fear index," has gone parabolic spiking over 45%.

And in an ominous sign for the global economy, commodity prices have plunged to their lowest levels since 1999 while crude oil has made a new post-financial crisis low. 

The selling is being attributed to any number of things, including China's slowdown, renewed uncertainty in Greece and the rest of the eurozone, the stronger dollar, the prospect of higher interest rates, stretched stock valuations, and the list goes on.