Clouds obscured the horseshoe-shaped crater left by the 1980 eruption, which blew off the cone-shaped mountain's top 1,300 feet, spawned mudflows, leveled hundreds of square miles of forest, and paralyzed towns and cities more than 250 miles away with volcanic ash.
The plume of ash that rose from the volcano eventually circled the globe.
"It seems to me this calls for a certain amount of humility, so let us commit to one another that we will wisely use this hard-won 25 years of insight to guide our future efforts to manage our natural resources," U.S. Agriculture Secretary Mike Johanns told visitors who gathered at a forest learning center near the volcano.
Washington Gov. Christine Gregoire said she was a young lawyer in the state attorney general's office at the time, on vacation with her family in rural Idaho. Without radio or television, their first clue to the eruption was the ash: "It was like a black curtain came down," she said.
There has been a renewed interest in Mount St. Helens in recent months, and not just because of the anniversary.
The volcano rumbled to life last fall for the first time since 1986, drawing thousands of visitors. Small earthquakes shook the crater and clouds of steam escaped thousands of feet into the air as magma moved toward the surface.
Scientists have said the mountain is rebuilding itself, just as plant and animal life have returned to the moonscape left by the 1980 eruption. Inside the crater, a lava dome that built up in the early 1980s was destroyed by last fall's activity, only to be replaced by one that is 500 feet taller and still growing.
For some visitors Wednesday, the anniversary was an occasion to remember what they were doing when the volcano blew its top.
"We looked at each other and thought it was the Second Coming, to tell you the truth," recalled Craig Reddinger, 48, who was sunbathing with his fiancee in Washington when the ash cloud blotted out the light.
For Todd Cullings, assistant director of the Johnston Ridge and Coldwater Ridge observatories, the anniversary was somber. He hiked out to the point where geologist David Johnston was killed after making a legendary radio call to his colleagues — "Vancouver! Vancouver! This is it!"
The ridge now named for Johnston was blasted clean of trees and soil in the eruption.
"I've spent the whole day reflecting on the power of this eruption," Cullings said. "It has opened people's eyes in a way that is really quite unique. You never look at the world the same way again. You can feel small and insignificant just to know the power."