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.
Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past.
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.
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.