https://www.engadget.com/2011-10-25-bluebiped-robot-needs-no-power-to-walk-for-miles-as-long-as-it.html
BlueBiped robot needs no power to walk for miles, as long as it's downhill (video)
Oct 25, 2011 - The robots; they're walking -- and this one's doing it under its own steam. This passive robotic frame requires no energy input, and is instead powered by its...