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A trader rubs his face while working on the floor of the New York Stock Exchange October 7, 2008 in New York City. - 

STEVE CHIOTAKIS: Not to dwell or anything on yesterday's massive sell-off. But we did see a five percent loss in value for the S & P and the Nasdaq -- and the Dow plummeted 512 points. That had a lot of folks scratching their heads.

Marketplace's Gregory Warner reports why it's so hard to ask why.

GREGORY WARNER: You sit there watching your 401K dribble down and so of course you're asking yourself: why? Why now did the market tank? So you turn on the TV:

MONTAGE: It's obvious that Europe is the story here. We're seeing banks liquidating -- back home, weekly unemployment claims showed little improvement -- fear just took over --we're hearing words like alarming, panic, devastating.

But what exactly were we so afraid of, yesterday?

As David Hefty of Hefty Wealth Partners points out.

DAVID HEFTY: I mean none of this stuff's new! We already know about debt in Greece. We already know about debt problems in the United States.

Yeah we know all that! So why does the market choose, Thursday, Aug. 4 to plunge 500 points?

Lawrence White is an economist at NYU Stern School of Business.

LAWRENCE WHITE: The markets are always trying to anticipate what's going to happen tomorrow. But also trying to figure out what do other people think is going to happen.

He says it's not that millions of investors all decided, yesterday, OK, time to panic. It's more like a lot more people thought that others would. The thing we have to fear is the fear of fear itself.

I'm Gregory Warner for Marketplace.

Follow Gregory Warner at @radiogrego