  By large-scale quantum Monte Carlo simulations we show that grain boundaries
in Helium-4 crystals are generically superfluid at low temperature, with a
transition temperature of the order of ~0.5K at the melting pressure;
insulating grain boundaries are found only for special orientations of the
grains. We also find that close vicinity to the melting line is not a necessary
condition for superfluid grain boundaries, and a grain boundary in direct
contact with the superfluid liquid at the melting curve is found to be
mechanically stable and the grain boundary superfluidity observed by Sasaki et
al. [Science 313, 1098 (2006)] is not just a crack filled with superfluid.
