From the Himalayas to the Alps and Rockies, mountain glaciers are rapidly melting. A sign of a warming climate, their retreat may also threaten hydropower and water supplies for cities below. To put current trends in context, scientists at Columbia's Lamont-Doherty Earth Observatory are studying rocks that record the ebb and flow of ice since the last ice age, over the past 20,000 years. They will travel the high peaks of the Peruvian Andes, including Nevado Coropuna, a 22,000 foot volcano. Other scientists will study pre-Columbian remains on the mountains. Geologist Gordon Bromley reports from Peru, while geochemist Gisela Winckler writes from the Lamont campus.