Studying ice chemistry may not sound very exciting - after all, isn't ice just frozen water? Almost true, but glaciers and ice sheets, along with snow, contain traces of everything in the atmosphere. All the impurities remain as long as the ice persists. The central parts of the Antarctic ice sheet are over 3 km thick. By drilling right through it, scientists have obtained layers of ice that are over 800 000 years old. Each layer contains a story about the composition of the atmosphere and the climate that the Earth was experiencing at that time. Extraction of this information requires painstaking analysis of the tiny quantities of impurities. A good understanding of the physical and chemical processes that take material from the atmosphere into the ice is also needed.