It was on the morning of May 18, this day in weather history, when Mount St. Helens was shaken by an earthquake of about 5.0 magnitude, and the entire north side of the summit began to slide down the mountain. The giant landslide of rock and ice, one of the largest recorded in history, was followed by and then actually overtaken by an enormous explosion of steam and volcanic gases, which surged northward along the ground at high speed. Learn more about your ad choices. Visit megaphone.fm/adchoices