don swanson: but when it erupted, the scale was far bigger than we anticipated. [music playing] on the morning of may 18, i was in vancouver. and suddenly, the traces on the seismograph started just moving back and forth, back and forth. george kourounis: the side of the mountain gave way. and what was once a beautiful, serene forest turned into hell on earth. that big bulge on the side of the volcano actually slid away. avalanche is the largest landslide in recorded history. michael poland: and as the volcano failed, it instantly exposed the magma that had been accumulating in the volcano itself to atmospheric pressure. and it's like shaking that can of soda and then opening it. all of the gas that was trapped in that magma suddenly came out. there wasn't a vertically directed explosion the way we typically think of a volcanic eruption. instead, it blasted out sideways. brad pitcher: so when this lateral blast occurred, it was this hot gas and ash and pulverized rock that traveled at speeds of 400, possibly even 500 miles per hour, creating what we call a pyrocl