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Ice forms at different rates and in different ways. Over time, the ice recrystallizes and traps small amounts of air, which vary according to environmental conditions. Since new layers form over the top of existing layers of ice, it is possible to obtain an accurate historical record of the past by drilling an ice core sample through a long-standing ice field. The best places to take these samples are in polar regions where the ice has been undisturbed for thousands of years.

One of the most famous examples of ice core drilling and analysis was the Greenland Ice Core Project. Visual records of the past stretching back hundreds of thousands of years have been obtained using this method. Such ice core samples may be several miles ...

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