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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.
The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen.
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.
Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales.