What are the odds of any of this actually happening? Probably low. But not zero, and also probably rising. National-security officials say obliquely that they have detected at least a couple significant efforts to cripple elements of the U.S. infrastructure system, without giving specifics. The Wall Street Journal has run several important stories on a series of mysterious attacks on electrical substations in California that left government officials deeply worried about the possibility of a “coordinated attack” on the grid that could darken whole cities or regions. And now, we’ve just seen the largest known effort to shut down the Internet. Maybe it’s nothing. But maybe it’s something.