  We demonstrate the instability of the free surface of a soft elastic solid
facing downwards. Experiments are carried out using a gel of constant density
$\rho$, shear modulus $\mu$, put in a rigid cylindrical dish of depth $h$. When
turned upside down, the free surface of the gel undergoes a normal outgoing
acceleration $g$. It remains perfectly flat for $\rho g h/\mu<\alpha^* $ with
$\alpha^*\simeq 6$, whereas a steady pattern spontaneously appears in the
opposite case. This phenomenon results from the interplay between the
gravitational energy and the elastic energy of deformation, which reduces the
Rayleigh waves celerity and vanishes it at the threshold.
