Thursday afternoon, the markets went on something of a joyride. Actually, it was more like a distress ride. After plummeting more than 900 points, the Dow Jones Industrial Average closed down about 3 percent. Why? It's unclear. CNBC said traders warned of a "black hole effect." A chief culprit, as Reuters pointed out, seemed to be fears of financial contagion from Greece, where residents are engaging in the ancient pastime of rioting.