OK before you all yell "EVAPORATION", I know that's the boilerplate answer, but why and how.
Back in high school we were taught water, and most other elements and compounds, have three states, solid, liquid, and gas. Which state it's in being dependent on temperature and pressure. OK that all makes sense.
To transition from one state to another sufficient energy has to be imparted to the substance to pass through that transition before it will continue to heat up. To get water from ice all the way up to a gas takes a lot of energy... ask all the distillers out there.
Still.. all good...
But my ice cubes disappear over time inside a closed freezer.
The water is evaporating, turning into a gas state, but where did the energy come from to take it all the way from a solid to a gas when it never left the freezer..