A U.S. Geological Survey climbing party ascends the Klutlan Glacier en route to 4766-m-high Mount Churchill. Their field studies led to the recent discovery that this volcano was the site of two of the most voluminous eruptions in North America in the past 2000 years. The White River Ash, two voluminous ash layers that extend across most of Canada's southern Yukon, originated from the summit caldera of Churchill volcano, which forms a twin volcano with 5005-m-high Mount Bona. The two peaks comprise the highest volcano in the United States. Photo by Game McGimsey (U.S. Geological Survey, Alaska Volcano Observatory).