Scientists are on high alert after spotting a key sign that a US volcano is about to erupt. Huge plumes of steam have been seen rising from Mount Spurr, one of Alaska's largest active volcanoes.

Scientists are on high alert after spotting a key sign that a US volcano is about to erupt. Huge plumes of steam have been seen rising from Mount Spurr, one of Alaska's largest active volcanoes.

This event would spew multiple plumes of ash rising as high as 50,000 feet into the air ... under 'advisory,' meaning it 'is exhibiting signs of elevated unrest.' 'We have been tracking ...