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 as much as 660 points. S&P futures are down 75 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…

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.”