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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.
In Greenland, a sequence of collaborative projects began in the 1970s with the Greenland Ice Sheet Project; there have been multiple follow-up projects, with the most recent, the East Greenland Ice-Core Project, expected to complete a deep core in east Greenland in 2020.
Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal.
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.
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.
As a result, alternating bands of lighter and darker ice can be seen in an ice core.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.The fluid must have a low kinematic viscosity to reduce tripping time (the time taken to pull the drilling equipment out of the hole and return it to the bottom of the hole).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.