In the wake of the devastating nuclear meltdown at Japan's Fukushima nuclear plant in 2011, officials have been hunting for new sources of green energy — and they're leaving no stone unturned. A heavy machinery maker in Japan called IHI Corp has successfully tested a prototype of a massive, airplane-sized turbine that can generate electricity from powerful deep sea ocean currents, Bloomberg reports, laying the groundwork for a promising new source of renewable energy that doesn't rely on sunny days or strong winds. The company's latest prototype is called Kairyu and weighs in at 330 tons. Two counter-rotating turbine fans...