The Dow finishes the day down 344 points - and for reasons that aren't entirely clear (aside from general worries about an economy that’s going nowhere fast). Traders are guessing at a few possibilities:

--Speculation that tomorrow's unemployment numbers for August will be stinkier than expected (the government reported that the number of Americans filing for unemployment benefits last week rose unexpectedly).

--Computerized trading models may have exacerbated the losses. Richard Sparks, an analyst at Schaeffer’s Investment Research, told the NYT that the S&P 500 index fell below 1,260 just minutes before the day’s biggest decline (some investors may be using computers that are programmed to sell stocks after that level was reached).

--Blame the dollar. The euro fell to $1.4354, down from about $1.60 in late July. That's good for the price of oil, which dropped $1.67 today, but it's mainly bad because stuff being exported overseas is getting more expensive - and the global economy is worsening. Exports had been one of the few areas propping up the economy. Here's the take at MarketBeat.