Mount St. Helens National Volcanic Monument, Washington © Don Geyer/Alamy

We"re standing on the Boundary Trail at Johnston Ridge in the Mount St. Helens National Volcanic Monument. This view of the volcano shows how the eruption of Mount St. Helens on May 18, 1980, ripped apart the once-conical summit, forever changing the Washington landscape. Forty years ago today, 110,000 acres within Gifford Pinchot National Forest were set aside to memorialize the deadliest and most destructive volcanic eruption in the United States.