Snow falls on ground and accumulates (gets deeper). As more snow falls, the bottom layers get compacted (thinner) because of the weight on top. In the end, only ice remains, and all air goes away. The deeper the ice, the older it is. Ice cores can be dated by looking at the layers: often every season can be seen in the ice core because it forms a layer. At other times it is more difficult to find out how old the ice is. Other methods can be used, like measuring conductivity or making mathematical models.