IT FINALLY HAPPENED: Dow 20,000. The iconic Dow Jones industrial average reached the momentous milestone. Thanks to a vigorous post-election stock market rally, the index has been flirting with the 20K mark since mid-December. But investors should be careful about celebrating the speed with which the venerable index got to 20,000: As this graphic shows, the last (and only) time the Dow jumped 1,000 points this fast, in 1999, it took 7½ years to level up again.