Explosions came out of nowhere. Substations, transformers, and transmission lines—some owned by electric cooperatives—were significantly damaged from bombs and attack drones, plunging millions of people into darkness.

Cyber attacks snarled information and communication systems, complicating recovery efforts.

Such was the mayhem confronting electric utility officials during GRIDEx III, the third biennial joint exercise of the power industry and the government to test the U.S. grid’s ability to sustain massive assaults.

Simulated widespread cyber and physical attacks during the exercise, held in late November 2015, took on heightened urgency in the wake of several incidents of real-world terrorism that month, including the downing of a Russian airliner in Egypt, the terror attacks in Paris, and another attack at a hotel in Mali.

“It made it all a little bit more real for us,” says David Revill, director of security and technology operations & chief technology officer for Georgia Transmission Corp, a transmission cooperative in Tucker, Ga. “We recognize that we have to stay diligent to keep our system secure.”

Staff at 14 G&Ts and four distribution cooperatives joined more than 4,000 participants from across the electric power industry, key federal and state agencies, and government officials from Mexico and Canada in the exercise led by the North American Electric Reliability Corporation.

The GRIDEx III exercise involved simulated destruction of communication systems on top of simulated grid devastation from explosive devices, drone warfare, and shootings. Social media also joined the mix for the first time. A simulated Twitter feed required coordinating teams to post accurate information to the public and ward off sabotage.

“We got to exercise more of that external communication muscle that we only get to simulate during our own internal exercises,” Revill says. “This was a really good opportunity to do it all at the same time with peers and organizations across the country.”

Read more on GRIDEx III on ECT.coop.