While Boston Dynamics has tested its quadruped robots outside before, it is much more difficult to build viable bipedal robots that can maintain their balance. Like humans, they have a much higher center of gravity, and are much more likely to lose their balance while trying to navigate obstacles that are found in outdoor terrain.

“Out in the world is just a totally different challenge than in the lab,” Marc Raibert, founder of Boston Dynamics, said at the Fab Lab Conference and Symposium (via Venture Beat). “You can’t predict what it’s going to be like.”

ATLAS is still learning to walk on its own, so it walks hesitantly and falters a little, like a young child. And like a child with training wheels, it still needs its power cord continually attached while it walks outside, so it needs to be under constant supervision.

But still, ATLAS was able to navigate a rugged hiking trail without falling down. This doesn’t sound like a huge step forward, but it is, especially considering that maintaining balance was arguably the biggest challenge for even the most advanced robots in the DARPA robotics challenge this year: