Two-dimensional projection of a screw dislocation in a hexagonal crystal of helium, as simulated by Boninsegni et al.  at a temperature of 0.25 K. This is a projection of a three-dimensional crystal that contains a dislocation along the vertical (z) direction. The density of blue points shows the regions that exhibit superfluidity inside the dislocation core. Red circles show the position of atoms (in the lower half-space only) and the dimensions are in Å. The whole picture is the result of a quantum Monte Carlo simulation performed with a particularly efficient algorithm.