Meet BALLU: a robotic system that uses buoyancy to stay stable. It has Helium filled balloons that make up its upper body. While the robot doesn’t float, it doesn’t fall either. The buoyancy force keeps the legs upright. This video shows a biped based on this concept with 2 degrees of freedom per leg. The knees are cable driven from the feet.
The robot can walk forwards, backwards, and move sideways. Dennis Hong has shared more info about this robot here.