The largest blackout that occurred in U.S. history was on August 14, 2003, leaving over 50 million people without power in the Northeast, Midwest and parts of Canada. The blackout’s primary cause was a software bug in the alarm system at a control room of the FirstEnergy Corporation in Ohio. Operators were unaware of the need to re-distribute power after overloaded transmission lines hit unpruned foliage. What would have been a manageable local blackout cascaded into widespread distress on the electric grid.