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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.
Ice cores have been studied since the early 20th century, and several cores were drilled as a result of the International Geophysical Year (1957–1958).
Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.
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.
In polar areas, the sun is visible day and night during the local summer and invisible all winter.
The bubbles disappear and the ice becomes more transparent.
The weight above makes deeper layers of ice thin and flow outwards.
Ice is lost at the edges of the glacier to icebergs, or to summer melting, and the overall shape of the glacier does not change much with time.
Impurities in the ice provide information on the environment from when they were deposited.
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The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, and the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide.