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This video shows a human-size bipedal robot, dubbed Mercury, which has passive ankles, thus relying solely on hip and knee actuation for balance. Unlike humans, having passive ankles forces Mercury to gain balance by continuously stepping. This capability is not only very difficult to accomplish but enables the robot to rapidly respond to disturbances like those produced when walking around humans. To achieve this capability, Mercury relies on an advanced inertial state estimation process and feedback control systems. The walking controller is based on whole-body control theory which enables precise trajectory tracking using the robot’s series elastic actuators. This means that the control methods of Mercury can be ported to many other humanoid robots with different morphologies and actuator setups. Finally, Mercury is a partial rebuild of the former Meka Hume biped, now including an Apptronik Medulla and Axon embedded nervous system. The legs have been partially redesigned and built to increase stiffness and incorporate the passive ankles. This research has been conducted at the Human Centered Robotics Lab at UT Austin.