Mount St. Helens’ 1980 eruption in southwestern Washington followed nearly two months of earthquakes, steam explosions, ash emissions, and a growing bulge on the volcano’s north flank. On May 18, a magnitude 5.1 earthquake triggered the largest subaerial landslide in recorded history, uncorking gas-rich magma and unleashing a lateral blast, a towering eruption column, widespread ashfall, and destructive lahars. Fifty-seven people were killed, hundreds of square miles were devastated, and damage exceeded $1 billion. The eruption transformed the mountain, reshaped Spirit Lake and nearby valleys, and led to preservation of the area as the Mount St. Helens National Volcanic Monument.