On a recent visit to Crystal Ice Cave in Idaho, climate and cave researchers had to wade through frigid, knee-deep water to reach the ice formations that give the cave its name. Cavers are good-humored about the hardships of underground exploration, but this water was chilling for more than one reason: it was carrying away some of the very clues they had come to study.

Ice is an invaluable source of information about the earth's past. Pollen trapped in ice from polar ice caps and mountaintop glaciers documents plant life up to 1.5 million years ago, and gas bubbles and water isotopes reveal glimpses of ancient temperatures.