Last April, unknown attackers shot up 17 transformers at a California substation in what the then-chairman of the Federal Energy Regulatory Commission Jon Wellinghoff called "the most significant incident of domestic terrorism involving the grid that has ever occurred" in this country.

Though news reports about the incident at the Metcalf transmission facility came out in April, The Wall Street Journal just pieced together the larger story of the attack together from regulatory filings and outside reporting.

Various power-grid facilities are vandalized or damaged regularly, but the details of this particular attack are startling.

Before the attackers opened fire on the transformers, fiber optic lines running nearby were cut.

Whoever executed the maneuver knew where to shoot the transformers. They aimed at the oil-cooling systems, causing them to leak oil and eventually overheat. By the time that happened, the attackers were long gone.

Wellinghoff toured the site with Navy Seals, according to the Journal, and they were convinced that it was a professional job. Several people in the Journal story join Wellinghoff in talking up the physical (not cyber) threats to the grid's stability.