Another plunge in the price of oil is driving stocks sharply lower.
Energy companies led the decline as crude dipped below $50 a barrel Monday.
Other stocks also fell as traders worried that the drop could indicate a weaker global economy and possibly lead to a dangerous, wider drop in other prices in the economy.
The Dow Jones industrial average lost 331 points, or 1.9 percent, to 17,501. It was down as much as 357 earlier.
The Standard & Poor's 500 fell 37 points, or 1.6 percent, to 2,024. The Nasdaq fell 56 points, or 1.2 percent, to 4,671.
Financial markets are starting the year with a stumble in stocks, oil and the euro. A plunge in the price of crude is hitting energy companies and leaving investors fearful that the selling will spread to other industries, dragging down indexes from their latest all-time highs reached a week ago.
Meanwhile, fears that Greece could drop out of the eurozone sent the euro to a nine-year low against the dollar.
KEEPING SCORE: The Standard & Poor's 500 index dropped 36 points, or 1.8 percent, to 2,022 as of 3:34 p.m. Eastern. The Dow Jones industrial average fell 332 points, or 1.9 percent, to 17,499. The Nasdaq composite slid 74 points, or 1.6 percent, to 4,652.
ENERGY: Oil prices extended a slide that began in June last year. Traders are worried that the world will be oversupplied with crude as demand slows along with the global economy.
Benchmark U.S. crude dropped $2.65 to close at $50.04 a barrel in New York. The price fell as low as $49.77, the first time it's gone below $50 in more than five years. Oil traded as high as $107 a barrel in June.
Energy stocks fell the most in the S&P 500, dropping 4 percent. Chevron fell $4.56, or 4.1 percent, to $108.02. Exxon Mobil dropped $2.64, or 2.8 percent, to $90.20.
EURO JITTERS: The euro also dropped. The currency was trading at $1.1945 after falling as low as $1.1862, its lowest since December 2005. The drop was triggered by reports that German Chancellor Angela Merkel no longer believes it would be too risky for the 19-member eurozone if Greece dropped out of the currency bloc. Upcoming elections in Greece could be won by the Syriza party which wants to renegotiate the terms of the country's international bailout, threatening its place in the euro group.
The euro has also been under pressure from expectations that the European Central Bank will expand its monetary stimulus as the region's economy struggles.
EUROPEAN STOCKS: France's CAC 40 fell 3.3 percent and Germany's DAX dropped 3 percent. Britain's FTSE 100 shed 2 percent. Greece's ATHEX index plunged 5.6 percent.
THE QUOTE: "The situation with Europe and Greece is causing some caution," said Scott Wren, Senior Global Equity Strategist at the Wells Fargo Investment Institute. "My feeling is that the ECB has band-aided the debt issue and we are going to continue to revisit the issue."
The S&P 500 has fallen 1.6 percent so far this year after ending 2014 close to record levels. Despite the sluggish start, Wren still thinks that U.S. stocks will return between 6 and 10 percent in 2015.
STOLEN DATA: Morgan Stanley slumped more than other banks after it said that an employee had "misappropriated" data from its wealth management clients. The bank said in a statement that an employee, who has since been fired, posted the account information of 900 clients on the internet. Morgan Stanley's stock dropped $1.35, or 3.5 percent, to $37.35.
METALS: Prices for precious and industrial metals ended mixed. Gold rose $17.80 to $1,204 an ounce, silver rose 45 cents to $16.21 an ounce and copper fell five cents to $2.77 a pound.
BOND PRICES: In U.S. government bond trading, prices rose. The yield on the benchmark 10-year Treasury note, which falls when prices rise, dropped to 2.04 percent.