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How are ice cores formed?Each year snow falls to the ground. As it builds up, it traps information about what was in the air at the time of the snowfall. As the layers thicken, the snow at the bottom is compressed into ice. The deeper you drill into the ice, the farther back into time you go. Anatomy of El Niño | Chasing El Niño | El Niño's ReachDispatches | Resources | Mail | Site Map | El Niño Home Editor's Picks | Previous Sites | Join Us/E-mail | TV/Web Schedule About NOVA | Teachers | Site Map | Shop | Jobs | Search | To print PBS Online | NOVA Online | WGBH © | Updated November 2000 |