  The observations of non-linear rotational susceptibility (NCRI) in samples of
solid He below 1-200 mK temperatures are conjectured to be describable in terms
of a rarified Gross-Pitaevskii superfluid of vacancies (or, more generally,
incommensuracies) with a transition temperature of about 50 mK, whose density
is locally enhanced by crystal imperfections. We argue that the observations
can be much affected by this density enhancement. We argue also that it is
likely that the ground state of every pure Bose solid is supersolid.
