The concept of harvesting the ocean as an energy source is nothing new, but in practice it's rarely utilized. That's beginning to change, though. This week, the first major underwater turbine was installed in Northern Ireland's Strangford Narrows—a body of water known for its fierce currents. SeaGen's twin blades measure 52 feet wide, and instead of intermittent winds, this green electricity generator will rely on the ever-changing tide to produce power for around 1,000 homes. Built by Marine Current Turbines, it will be operational this summer.