When the power went out in New York City and elsewhere inside a triangle bounded by Ohio, Massachusetts, and Ontario in August 2003, many of us learned that America's power grid is out of date and vulnerable to massive failure. When an estimated 55 million people lost electricity in the U.S. and Canada because of a fallen limb and a software bug at a power plant near Cleveland, we learned a new phrase: "smart grid." It seemed clear we should upgrade to one.

That still holds true — a smart grid will improve the efficiency, reliability, and, yes, security of our electricity infrastructure. But maybe things aren't so dire with the three grids we have now — covering the Eastern U.S., Texas, and the West.

Take the Hollywood-worthy attack on a California power substation last April. It started at just before 1 a.m. on April 16, at PG&E's Metcalf Transmission Substation outside San Jose. The attackers severed underground fiber-optic telephone cables then, at 1:30 a.m., opened fire for 19 minutes, carefully shooting out and destroying 17 of the 23 giant transformers that send power to Silicon Valley. Then, a minute before police arrived, the attackers disappeared. No arrests have been made. The FBI appears stumped.

The damage to the plant — $15.4 million worth — was so severe it took workers 27 days to get the substation back up and running. And yet, nobody lost power and, until the former head of the Federal Energy Regulatory Commission (FERC) came forward on Wednesday, almost nobody in California or elsewhere knew it even happened. PG&E simply rerouted power around the substation until it was back up and running.

Still, for Jon Wellinghoff, the FERC chairman during the incident (he stepped down in November), the attack is a shot across the bow, a professional hit job, perhaps a trial run for a larger attack, and "the most significant incident of domestic terrorism involving the grid that has ever occurred" in the U.S., he tells The Wall Street Journal. "A coordinated attack could put this country in a world of hurt for a long time," he added to the San Jose Mercury News.

The situation Wellinghoff lays out is frightening: A terrorist group used the Metcalf hit as a dry run for synchronized attacks on some number of substations around the country that would destabilize the grid and black-out most of the country. It would take a surprisingly small number of substations, he added, citing a FERC analysis.

Transmission substations are critical links in the grid, ramping up voltages to send electricity long distances over high-voltage wires and reducing power levels to send it out for local use. The Wall Street Journal's Jason Bellini has a helpful primer on the U.S. power grid:

But the FBI doesn't believe the Metcalf attack was terrorism, and it failed to shut out any lights in Silicon Valley. Few of the 274 significant instances of vandalism or deliberate damage at U.S. power plants over the past three years have had any impact, either. Nor have the 700 weather-related problems tallied by The Wall Street Journal.

"I don't want to downplay the scenario [Wellinghoff] describes," Gerry Cauley, president of the North American Electric Reliability Corporation, tells The Wall Street Journal. "I'll agree it's possible from a technical assessment," but the chances the saboteurs would shut off power for more than a few hours are pretty small. "It’s harder to knock out the lights than people think because of redundancy and resilience," he tells The New York Times.

Norimitsu Onishi and Matthew L. Wald at The Times take a deeper look at the obstacles to blacking-out the U.S. by knocking out a critical number of substations:

Most of the substations are owned by publicly traded utilities; a few are owned by government agencies.... The location of substations is public, but it is a closely guarded secret what combination of them would have to be knocked out to cause extensive harm. It could be as few as a handful in each of the three grids, the eastern continent, from Halifax to New Orleans, the western continent, from New Mexico to Vancouver, and Texas. [New York Times]

And the Metcalf attack did spook the nation's power utilities into action. They are in the middle of a two-and-a-half-year evaluation of which substations or combination of substations are most critical to keeping the nation's power on, coming up with contingency plans in case those facilities are knocked out, and proposals on how to reroute power after an attack to provide power to the most people possible, The New York Times reports.

America's power grids are surely vulnerable to a concerted terrorist attack, cyber attacks, or series of unfortunate accidents. But they're less fragile that many would have thought when the lights went out in New York City.