After a month of historic gains, the U.S. stock market ended October on a low note over concerns about Europe's plan to contain Greece's debt crisis.
On Monday, the Dow Jones industrial average tumbled 276 points to close at 11,955.
"We're obviously very sensitive to the European markets," Stephen Leuer, a floor trader at X-FA Trading, told the Wall Street Journal.
October has historically been a bad month for investors. The crash of 1929, 1987's Black Monday, and the financial meltdown in 2008 all took place during October.
This year, however, was the exception. The Dow Jones industrial average rose more than 9.5 percent, its biggest monthly gain in its 115-year history.
And the Standard & Poor's 500 futures rose more than 10 percent, its biggest gain since 1991.
But news that Greek Prime Minister George Papandreou called for a popular referendum on the European Union debt deal seemed to plant doubt in the hearts of investors.
"Maybe the deal is in jeopardy or maybe it's simply not a sure thing. Any sort of doubt is definitely going to set us back," Leur said.