In northern California last spring, someone slipped into an underground vault near a busy freeway and cut telephone cables. Within half an hour, says the Wall Street Journal, snipers opened fire on an electrical substation. Shooting for 19 minutes, they surgically knocked out 17 giant transformers that funnel power to Silicon Valley. A minute before a police car arrived, the shooters disappeared into the night. It took utility workers 27 days to bring the substation back to life. Nobody has been arrested in the attack at PG&E Corp.’s Metcalf transmission substation. It is an incident of which few Americans are aware. An ex-federal regulator is calling it a terrorist act that, if widely replicated, could take down the U.S. electric grid and black out much of the country. It was “the most significant incident of domestic terrorism involving the grid that has ever occurred” in the U.S., said Jon Wellinghoff, then-chairman of the Federal Energy Regulatory Commission. The Journal assembled a chronology of the attack. “This wasn’t an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation,” Mark Johnson, a retired PG&E official, told a utility security conference.