[Dr. Scott W. Tinker] Hydro is the ideal electricity resource, and that's why we've developed pretty much all of it. To begin with, the energy comes from something we'll never run out of: gravity. Weather moves water uphill, and gravity brings it down through the turbines. And voila, electricity. Lots of it. The biggest power plants in the world are dams, and they can spin up the turbines almost instantly whenever we need the power. This means hydro can be always-on base load power, or it can follow electricity demand by the minute, all with zero emissions. And although the plants are expensive and high-impact to build, they're cheap to operate, and may last for 100 years. But the water may not. As population and development increase, so do water demands for other things, like agriculture. And climate change could bring more droughts and hotter summers. Maybe more important, the places to build hydropower are limited. In the developed world, we've nearly used them all. The big projects in the future will be in China, India, and the developing world. But there, as everywhere, damming a river has environmental, social, and economic impacts that not everyone is ready to bear. For these reasons, as global energy demands rise, hydro will supply a shrinking percentage of it, and the growth of this clean, flexible resource will mostly be in small local projects and refitting old ones with better equipment. So that's hydro. It's phenomenal, but we won't see much more of it. For a spectacular hydro site visit, see the film.