Four microscopic liquid channels, each about 100 microns across (roughly 1/250th of an inch), come together at the corner where four ice grains meet. Because impurities are concentrated in the channels, they stay liquid even though the temperature is below freezing – for the same reason that ice melts on the sidewalk when you throw salt on it. (Note 200 micron scale in bottom left corner.

(Photo credit: Heidi Mader, University of Bristol)