U.S. stocks plunge to worst 1-day drop since 2001 - DOW Closes Down 415 Points Today!

Following weakness last week, the blue-chip average has now erased all its gains for the year so far.

After sliding all day, U.S. stocks seemed to fall off a cliff in the afternoon, as data providers failed to keep up with selling programs, noted Stephen Sachs, head of trading at Rydex Investments.

"There's not even a flight to quality into gold or the Swiss franc, which tells me that we're closer to the beginning than to the end of this," Sachs said.

At about 3 p.m., the drop in the Dow accelerated and in the blink of an eye was down 546.20 points because of a price tabulation error at Dow Jones, which calculates the average in real-time and transmits the figure to market-data providers. According to Dow Jones, there was a temporary lag in its calculation and when a backup computer came on line, the displayed average instantly spiked lower to catch up with the current level.

Trading volumes hit record levels on the New York Stock Exchange, where more than 2.3 billion shares exchanged hands. More than 3 billion shares traded on the tech-heavy Nasdaq stock market.

Declining issues outpaced gainers by 29 to 4 on the NYSE and by 14 to 1 on the Nasdaq.

The S&P 500 index ($SPX : S&P 500 Index dropped 50 points, or 3.5%, to 1,399, while the technology-heavy Nasdaq Composite (COMP :

Nasdaq Composite Index took the brunt of the selling, falling 96 points, or 3.9%, to 2,407. Both indexes put in their worst one-day point declines since September 2001.

"Whether it's contained to today or if it's the beginning of a more serious correction, we'll have to see over the next couple of days," said Paul Nolte, director of investments at Hinsdale Associates.