At a little after 11, the Dow was down more than 200 points. As is often the case during really bad selloffs, there's no single explanation. Some analysts will point to a market plunge in China or Vice President Cheney being the apparent target of a Taliban bombing attack in Afghanistan or a worse-than-expected durable goods report. But many on Wall Street had become convinced that after an eight-month rally, the market was due for a correction - no matter what the news of the day happened to be. The big question, of course, is how big a pullback? Harry Clark, CEO of Clark Capital Management, tells CNNMoney.com that after a decline of 4 or 5 percent it will be done. Others believe the pushback could be closer to 10 percent.

**Update: With about an hour of trading still left, the Dow was down 500 points. That came after the index plunged 200 points almost instantly. Now it's recovering, off around 350 points (there's still more than a half hour left). That 200-point instant plunge could be one of the big stories of the day.

Updates throughout the afternoon.