  We explore the intrinsic dynamics of spherical shells immersed in a fluid in
the vicinity of their buckled state, through experiments and 3D axisymmetric
simulations. The results are supported by a theoretical model that accurately
describes the buckled shell as a two-variable-only oscillator. We quantify the
effective "softening" of shells above the buckling threshold, as observed in
recent experiments on interactions between encapsulated microbubbles and
acoustic waves. The main dissipation mechanism in the neighboring fluid is also
evidenced.
