The most powerful volcano eruption of the 20th century occurred June 6, 1912, when Novarupta Volcano in Alaska blasted a huge cloud of ash into the sky with a roar that could be heard more than 750 miles away in Juneau—one hour after the eruption began. The ash blanketed Kodiak Island, about 100 miles away, for the next three days, covering the town there with ash a foot deep, completely blocking the sun, and causing buildings to collapse from the weight of the ash on their roofs.

Experts say an eruption on the same scale today would ground commercial air travel across North America, due to the damage that volcanic ash could cause to jet engines, windshields, and external parts.