  We investigate the dynamics of a magnetoelastic robot with a dipolar magnetic
head and a slender elastic body as it performs undulatory strokes and burrows
through water-saturated granular beds. The robot is actuated by an oscillating
magnetic field and moves forward when the stroke amplitude increases above a
critical threshold. By visualizing the medium, we show that the undulating body
fluidizes the bed, resulting in the appearance of a dynamic burrow, which
rapidly closes in behind the moving robot as the medium loses energy. We
investigate the applicability of Lighthill's elongated body theory of fish
locomotion, and estimate the contribution of thrust generated by the undulating
body and the drag incorporating the granular volume fraction-dependent
effective viscosity of the medium. The projected speeds are found to be
consistent with the measured speeds over a range of frequencies and amplitudes
above the onset of forward motion. However, systematic deviations are found to
grow with increasing driving, pointing to a need for further sophisticated
modelling of the medium-structure interactions.
