For years, security experts have been fearing "The Big One." Not terrorist incidents, bombs, or hurricanes--their warnings have been centered on cyberattacks. And now, one of the largest and potentially most damaging data breaches ever has occurred, at the credit-reporting company Equifax. The company revealed that personal information from about 143 million consumers--possibly including Social Security numbers, addresses, and even credit card numbers--was stolen. The scale of this breach is hard to overestimate. It could ultimately affect more than half of the adult population of the United States.