I recently stood next to an electrical generator, big enough to power a city the size of Seattle (about 1,000 megawatts, known as a gigawatt). It was surprisingly small, no larger than a classroom with a tall ceiling.

      The generator’s spinning shaft could be seen where it connected to the steam turbine, next in line. And backing it up were three more turbines, helping to keep that long shaft spinning at 1,800 revolutions every minute.

      The generator doesn’t spin freely because every electrical light and appliance in that gigawatt-sized city is resisting it. It takes a lot of push from the four steam turbines to keep it up to speed. Some power plants create the steam in a boiler heated by burning coal, others by using nuclear fission of uranium-235 to generate the requisite heat. The cleanest method of all is harvesting steam from water sprayed on hot granite a few miles [5 km] underground.