The global market bloodletting continues as the sell-off has officially reached historic levels. 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 are down 500 points. S&P futures are down 53 points. Nasdaq futures are down 186. 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. The selling is being attributed to any number of things, perhaps including China's slowdown, renewed uncertainty in Greece and the rest of the eurozone, the stronger dollar, the prospect of higher interest rates, stretched valuation, etc. Leading the risks cited is the prospect for tighter monetary policy from the central banks, which Bank of America Merrill Lynch dubbed the "ultimate risk." "We've long felt that the only thing preventing another financial crisis has been extraordinary central bank liquidity and general interventions from the global authorities which we still expect to continue for a long while yet," Deutsche Bank's Jim Reid wrote on Monday. "So when policy changes, risks arise. The genesis of this recent sell-off has been the threat of the Fed raising rates next month, but China's confrontational move two weeks ago and the subsequent knock-on through EM have accelerated us towards something more serious." Even the longer-term optimists acknowledge that things could get darker before the dawn. "No Elizabeth, this isn't 'The Big One' and while the worst may not be over, it is time to focus on adding risk rather than selling," Brean Capital's Peter Tchir wrote. Join the conversation about this story » NOW WATCH: RED EVERYWHERE: It’s a global market meltdown