Georgia Tech researchers led by Daniel Goldman used high-speed X-ray imaging to watch sandfish beneath the surface. After a brief burst of limb-driven digging to submerge, the animal folds its legs back and generates a single-frequency undulation that travels from head to tail. Forward speed measured in the region of ten centimetres a second, faster than the animal's above-ground running in some trials.
Dry sand behaves as a granular medium, neither solid nor fluid, and the physics of swimming through it differs from swimming in water. Modelling the sandfish as a swimmer in a frictional fluid reproduced the observed relationship between wave amplitude and forward speed, and the work fed directly into the design of sand-swimming robots.
The body is built for the medium: overlapping smooth scales with low friction, a shovel-shaped snout, valve-like nostrils and a countersunk jaw that keeps sand out. Whether the low friction of the scales is chiefly about reducing drag or about resisting abrasion has been argued in the materials literature, and the answers are not exclusive.