Stocks tumbled on Tuesday after investors were shocked by a surprise call for a Greek referendum on the EU bailout plan.
The S&P 500 has slid more than 5 percent so far this week in moves reminiscent of the market swings seen over the past two months and after investors thought the worst of the euro zone debt crisis was over.
The plummet in shares comes after stocks rebounded to post their best month in 20 years in October. The gains were fueled by hope for an eventual deal to rescue Greece, finally agreed upon at last week's European Union summit and in doubt due to the proposed referendum.
Some commentators think Greece may be headed for a larger default than under the plan and ultimately an exit from the Euro.
