From MIT:

Further info from Evan Ackerman at IEEE Spectrum:

We were wrong: it's not running untethered, it's bounding untethered. And unconstrained. And outdoors! 

Two things strike us as particularly amazing about this: the first thing is that it's quiet, powered by electric motors and batteries. We've come to expect that compact systems capable of delivering high amounts of power rely on liquid fuels and hydraulics, because that's how you get the most power density: it's why Boston Dynamics uses gasoline engines to power hydraulic pumps on all of its dynamic robots. Also, high torque electric motors (like you'd need to get a robot to jump) have a tendency to overheat and destroy themselves, but MIT seems to have solved all of these issues, since they have a bounding, battery-powered robot that works. We're not sure yet how long it works for, but it works... (cont'd)