The Dow Jones Industrial Average was down about 520 points at the closing bell, falling to 10,719. The Dow hasn't been this low since last September.
Analysts say the ongoing European debt crisis and disappointing news on the jobs front are fueling investor anxiety.
Today's frantic trading pace erased many of yesterday's gains.
The Nasdaq lost about 100 points, and the S&P 500 was down 52 points.