Energy companies led the way down today. Chevron fell 6 per cent, the most in the Dow Jones industrial average.
The price of US crude oil sank 7 per cent and is trading at its lowest level since May 2003.
The Dow was down 422 points, or 2.6 per cent, to 15,590 as of 11:45 am local time.
The Standard & Poor's 500 index dropped 52 points, or 2.8 per cent, to 1,828, its lowest level in more than a year.
The Nasdaq composite lost 122 points, or 2.7 per cent, to 4,353.
Bond prices rose as investors shifted money out of stocks.