  Consider at a finite temperature T a superfluid moving with a velocity v
relative to the thermal bath or its normal component. From Landau's argument
there exists a critical v_c (T) beyond which excitations can be spontaneously
generated and the system becomes unstable. Identifying the final state induced
by such an instability has been an outstanding open question. Using holographic
duality we perform dynamical simulations of evolutions from initial unstable
states, and find that the system settles to a homogenous superfluid state with
a final velocity below the critical velocity. The dynamical evolution process
appears to be highly chaotic, exhibiting transient turbulence. Nevertheless we
are able to identify from the simulations a universal physical mechanism for
the reduction of superfluid velocity, in terms of spontaneous nucleation of
solitons. We also derive a simple analytic formula which relates the final
velocity to the number of solitons nucleated during the evolution.
