The ice caves of the Vatnajokull glacier in Iceland are unique.  Every year this glacier changes shape, and it is only exposed for a few weeks at the end of every year.

It is something to behold: several caves are exposed, often full of bright blue ice. The bluer the ice is, the older it will be.

Have a look at these photos by Mikael Buck:

More information:
www.extremeiceland.is/ice-caves
localguide.is