  The macroscopic flow properties of pure He II are probed in the limit of zero
temperature using an oscillating grid. With increasing oscillation amplitude
the initially pure superflow changes abruptly: the resonant frequency decreases
and the response becomes strongly nonlinear, attributable to the growth of a
boundary layer of quantized vortices that increases the effective mass of the
grid. On further increase of oscillation amplitude, the flow undergoes a
transition to turbulence.
