  Turbulent flow restricted to two dimensions can spontaneously develop order
on large scales, defying entropy expectations and in sharp contrast with
turbulence in three dimensions where nonlinear turbulent processes act to
destroy large-scale order. In this work we report the observation of unusual
turbulent behavior in steady-state flow of superfluid \4He---a liquid with
vanishing viscosity and discrete vorticity---in a nearly two-dimensional
channel. Surprisingly, for a range of experimental parameters, turbulence is
observed to exist in two bistable states. This bistability can be well
explained by the appearance of large-scale regions of flow of opposite
vorticity.
