"The largest eruption on Earth this century occurred at Novarupta Volcano, Alaska, in June 1912, creating Katmai Caldera and the Valley of Ten Thousand Smokes. Volcanic ash (more than from all other historical eruptions in Alaska combined) devastated areas hundreds of miles away. Such massive eruptions will occur again in southern Alaska, threatening its rapidly growing population. To protect the public, U.S. Geological Survey (USGS) and other scientists with the Alaska Volcano Observatory closely monitor the State's many active volcanoes."
USGS Fact Sheet-075-98
Alaska Volcano Observatory: http://www.avo.alaska.edu/