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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.
At Summit Camp in Greenland, the depth is 77 m and the ice is 230 years old; at Dome C in Antarctica the depth is 95 m and the age 2500 years.
Numerous other deep cores in the Antarctic have been completed over the years, including the West Antarctic Ice Sheet project, and cores managed by the British Antarctic Survey and the International Trans-Antarctic Scientific Expedition.
These include soot, ash, and other types of particle from forest fires and volcanoes; isotopes such as beryllium-10 created by cosmic rays; micrometeorites; and pollen.
The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen.
Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.
The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core.