As shown on the History Channel. Over 20 years ago, Mount St. Helen's--thought to be dormant--shocked America when it exploded. It is an acidic volcano--the magma beneath is full of volatiles making it highly explosive. A new plug has formed in its throat and is rising. When it blows, it will be like uncorking a champagne bottle, releasing pressure below and allowing dissolved gases to escape and explode. The question is, When will it blow again?