A long cylinder of ice that is obtained by drilling vertically downwards through an ice sheet. The seasonal accumulation of ice produces bands by which the age of the core can be determined and sections of it dated. Analysis of ice from particular sections reveals information regarding past climate (see oxygen-isotope analysis). Dust trapped in the ice indicates arid conditions (and low temperatures) and bubbles of air record the chemical composition of the atmosphere. Cores taken from the Greenland ice sheet (the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project (GISP) have obtained samples up to 20 000 years old. Cores drilled near the Vostok Station, Antarctica, have reached ages of more than 40 000 years.