  Heat is carried in superfluid He-4 by the motion of the normal fluid$^{1}$, a
counterflowing superfluid component serving to eliminate any net mass flow. It
has been known for many years that above a critical heat current the superfluid
component in this counterflow becomes turbulent. This turbulence takes the form
of a disorganized tangle of quantized vortex lines and is maintained by the
relative motion of the two fluids$^{2-3}$. It has been suspected that the
normal fluid may also become turbulent$^{4}$, but experimental verification is
difficult without a technique for visualizing the flow. Here we report a series
of visualization studies on the normal-fluid component in a thermal counterflow
performed by imaging the motion of seeded metastable helium molecules using a
laser-induced-fluorescence technique$^{5-8}$. We present evidence that the flow
of the normal fluid is indeed turbulent, at least at relatively large
velocities. Thermal counterflow in which both components are turbulent presents
us with a new and theoretically challenging type of turbulent behaviour.
