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An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier.Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, and an ice core contains ice formed over a range of years.Soviet ice drilling projects in Antarctica include decades of work at Vostok Station, with the deepest core reaching 3769 m.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.Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice.

Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past.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.As further layers build up, the pressure increases, and at about 1500 m the crystal structure of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a clathrate.Ice cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface.Early cores were often collected with hand augers and they are still used for short holes.

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