Thin section of Antarctic ice (from a depth of 87 meters) observed under polarized light; the variety of colors reflects different orientations of the ice crystals. Air bubbles (black, about 1 millimeter across) provide a tiny record of ancient atmospheres that can be preserved in the ice for more than 100,000 years. See page 926. Other aspects of atmospheric evolution are discussed in the special section beginning on page 905. [Photograph: Michel Creseveur]

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