  Legged locomotion on flowing ground ({\em e.g.} granular media) is unlike
locomotion on hard ground because feet experience both solid- and fluid-like
forces during surface penetration. Recent bio-inspired legged robots display
speed relative to body size on hard ground comparable to high performing
organisms like cockroaches but suffer significant performance loss on flowing
materials like sand. In laboratory experiments we study the performance (speed)
of a small (2.3 kg) six-legged robot, SandBot, as it runs on a bed of granular
media (1 mm poppy seeds). For an alternating tripod gait on the granular bed,
standard gait control parameters achieve speeds at best two orders of magnitude
smaller than the 2 body lengths/s ($\approx 60$ cm/s) for motion on hard
ground. However, empirical adjustment of these control parameters away from the
hard ground settings, restores good performance, yielding top speeds of 30
cm/s. Robot speed depends sensitively on the packing fraction $\phi$ and the
limb frequency $\omega$, and a dramatic transition from rotary walking to slow
swimming occurs when $\phi$ becomes small enough and/or $\omega$ large enough.
We propose a kinematic model of the rotary walking mode based on generic
features of penetration and slip of a curved limb in granular media. The model
captures the dependence of robot speed on limb frequency and the transition
between walking and swimming modes but highlights the need for a deeper
understanding of the physics of granular media.
