EMP, something discovered in the late 1950s. Consider a nuclear bomb. Several tons of uranium or plutonium and casework and whatever. Detonate it. The furious energy of the atomic detonation blows all the electrons off the atoms comprising the bomb, and immediate surroundings. Turns them into charged ions, and blows them away at supersonic speed. This is a massive moving electric charge, which creates a massive magnetic field that spreads out from the detonation site at the speed of light. Such a field will induce massive electric currents into any conductor that it encounters. Other doomsayers worry that solar flares or "coronal mass ejections" can do the same thing. They point to the 1850's Carrington event, a solar flare so strong that telegraph wires sizzled and crackled with sparks in telegraph offices, scaring the bejezus out of telegraph operators.
The fear is, that such currents will melt wires, arc over insulators, trip circuit breakers, melt transformers and destroy alternators. Wreaking the electric power grid, the internet, the wired phone system, the cell phone system, stereos, TVs, Ipads, everything electric or electronic and hurl our civilization back into a dark age, lit only by fire.
Not to worry. The millions of miles of wire hanging from poles, all across the continent, get struck by lightning, every minute of every day. A single lightning strike stresses electrical systems to the limit. Lightning will arc over any insulator, and fry anything. Half a century ago, every summer lightning storm would knock out the electric power. Well, over the half century since then, the power companies have hardened their systems. My lights stay on, unless a windstorm drops a tree on the wires and breaks them. Local damage, sure. My mother's home took a lightning hit a few summers ago. Blew out her satellite receiver, her DVD player, and few other things. But the electric lights survived, along with the furnace, the hot water heater, and the electric stove.
I don't believe any EMP event will ever be as bad as a lightning strike. We have hardened every thing against lightning strikes. They are probably safe against EMP events, be they hostile nukes, or solar flares.