Markets Plunge as Oil Dips Below $50; Euro Drops on Greece

Stocks had already endured a weak open amid concerns that Greece could exit the euro, adding to worries about the poor outlook for growth in that region. As oil continued to slide, the losses deepened and the Standard & Poor's 500 index ended the day with its biggest loss in three months.

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"The lower that oil prices go, the more it reinforces into the market's mind that perhaps this is more of a demand issue than a supply issue," said Burt White, Chief Investment Officer at LPL Financial. That raises questions "about the robustness of this recovery."