  We describe and model experimental results on the dynamics of a "ludion" - a
neutrally buoyant body - immersed in a layer of stably stratified salt water.
By oscillating a piston inside a cylinder communicating with a narrow (in one
of its horizontal dimensions) vessel containing the stably stratified layer of
salt water, it is easy to periodically vary the hydrostatic pressure of the
fluid. The ludion or Cartesian diver, initially positioned at its equilibrium
height and free to move horizontally, can then oscillate vertically when forced
by the pressure oscillations. Depending on the ratio of the forcing frequency
to the Brunt-V\"{a}is\"{a}l\"{a} frequency of the stratified fluid, the ludion
can emit its own internal gravity waves that we measure by a classical Particle
Image Velocimetry technique. Our experimental results describe first the
resonance of the vertical motions of the ludion when excited at different
frequencies. A theoretical oscillator model is then derived taking into account
added mass and added friction coefficients and its predictions are compared to
the experimental data. Then, for the larger oscillation amplitudes, we observe
and describe a bifurcation towards free horizontal motions. Although the
internal gravity wave frequencies are affected by the Doppler shift induced by
the horizontal displacement velocities, it seems that, contrary to surface
waves associated with Couder walkers they are not the cause of the horizontal
swimming. This does not however, exclude possible interactions between the
ludion and internal gravity waves and possible hydrodynamic quantum analogies
to be explored in the future.
