  When submitted to large stresses at high temperature, usual crystals may
irreversibly deform. This phenomenon is known as plasticity and it is due to
the motion of crystal defects such as dislocations. We have discovered that, in
the absence of impurities and in the zero temperature limit, helium 4 crystals
present a giant plasticity that is anisotropic and reversible. Direct
measurements on oriented single crystals show that their resistance to shear
nearly vanishes in one particular direction because dislocations glide freely
parallel to the basal planes of the hexagonal structure. This plasticity
disappears as soon as traces of helium 3 impurities bind to the dislocations or
if their motion is damped by collisions with thermal phonons.
