In the 1960s, scientists began extracting ice cores from Greenland and Antarctica. These frozen columns can compress hundreds of thousands of years into their vertical reach.

Like the clamshells, ice cores tell scientists about the temperature record through their chemical composition, particularly the oxygen isotopes--ice with less heavy oxygen (made heavy by additional neutrons) indicates cooler temperatures. But ice cores can do so much more. The thickness of layers reveals how much snow accumulated in a year. If there's a large different in snow for the same year between one ice core and another, that can indicate which way the wind was blowing. And tiny air bubbles trapped in the ice can allow scientists to see what gases were present in the ancient atmosphere.

No, this picture doesn't show the warehouse where they're keeping the Ark of the Covenant; it's the US Geological Survey National Ice Core Lab.