Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.As a result, alternating bands of lighter and darker ice can be seen in an ice core.
It can be up to about 20 m thick, and though it has scientific value (for example, it may contain subglacial microbial populations), Cores are often drilled in areas such as Antarctica and central Greenland where the temperature is almost never warm enough to cause melting, but the summer sun can still alter the snow.
In polar areas, the sun is visible day and night during the local summer and invisible all winter.
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice.
Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales.
The cuttings (chips of ice cut away by the drill) must be drawn up the hole and disposed of or they will reduce the cutting efficiency of the drill.