It slowly turns to ice, compressed by its own weight. Thereby enabling us to get https://en.wikipedia.org/wiki/Ice_core https://en.wikipedia.org/wiki/Ice_core

It slowly turns to ice, compressed by its own weight. Thereby enabling us to get https://en.wikipedia.org/wiki/Ice_core https://en.wikipedia.org/wiki/Ice_core and analyse the layers like tree rings, err... https://en.wikipedia.org/wiki/Dendrochronology https://en.wikipedia.org/wiki/Dendrochronology