Sublimation
Sublimation is a physical process where a substance (water in this case) changes from a solid directly to a gas and skips the liquid phase. This occurs at temperatures below freezing and is driven by steep vapor pressure gradients in dry air. This process happens rather slowly but continuously within snow pack and also in an automatically defrosting freezer. Here, the ice cubes in the tray originally formed with equal and uniformly horizontal surfaces. When neglected in the freezer for months, ice on the surfaces sublimates and the volume of each cube is reduced. The patterns of this volume loss are irregular and dependent on airflow within the freezer, so non-horizontal surfaces are generated and the ice cubes are no longer identical to one another.
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