A Deep beneath the ice sheets of Antarctica and Greenland lies a record of the earth's climate stretching back hundreds of thousands of years.
B Each year, snow falls and is pressed into ice, trapping tiny bubbles of ancient air. By drilling long cylinders of ice, scientists can measure the gases in these bubbles.
C The cores show that carbon dioxide levels and temperature have risen and fallen together over many cycles. They also contain dust and ash from distant volcanoes.
D Drilling is demanding work. Teams live in remote camps for months, and the cores must be kept frozen during transport to laboratories.
E One project aims to recover ice more than a million years old. Researchers hope it will explain why the timing of ice ages changed in the past. The results may help improve predictions of future warming.