The 24-megaton blast left behind a time machine.

By scorching and smothering a Northwest mountaintop into a primordial state, the 1980 eruption of Mount St. Helens burst open a portal to strange, hostile conditions.

On Eruption Day Plus 15 Years, this vast laboratory continues to evolve, yielding insights on everything from the origins of life to managing forests.

“If you made a grant proposal to have 250,000 acres of forest land denuded of life so you could study what happened next, they’d think you were crazy,” said Jim Quiring, director of the Coldwater Ridge Visitor Center. “Yet that’s what nature provided here.”

What nature also provided was a time machine for people who are NOT scientists.

Residents of the Pacific Northwest tend to remember what they were doing at 8:32 a.m. on the Sunday morning of May 18, 1980. They watch the anniversaries roll by - fifth, 10th, 15th - and can measure their own evolution.

Jenny Butler was 4 and playing in her back yard at Silver Lake when the mountain erupted 38 miles away. The ash seemed like snow - fun stuff in which to play - and she resisted her mom’s calls to come inside.

Throughout her school years in nearby Toutle, Butler got used to answering questions about the volcano from the stream of tourists coming to town.

This spring, Mount St. Helens is a job to Butler. She sells soft-serve cones at a 2-year-old snack bar at Coldwater Ridge with a stunning crater view. It is one of several newer facilities - including two opening today - that make it easier than ever for visitors to explore the volcano.

But whether one goes there or not, the passage of 15 years makes Mount St. Helens part of what defines generations in the Pacific Northwest.

“I don’t think of it as special,” Butler said. “It’s just always been there.”

Explorer William Lewis described St. Helens in 1806 as “a kind of cone in the form of a Sugar lofe … the most noble looking object of its kind in nature.”

Other admirers followed, but during the next half-century they occasionally witnessed the cone spurting ash and steam. Then, after an 1857 eruption, Mount St. Helens slumbered for 123 years.

Spirit Lake to its north beckoned generations of Scouts and anglers. Mountaineers challenged its 9,677-foot peak. Loggers cut their way through nearby ridges and valleys.

In short, St. Helens was a setting for typical Northwest life.

The wake-up call was a 4.1-magnitude earthquake on March 20, 1980. A week later, the mountain spewed ash and steam a mile into the air - the first eruption in the contiguous 48 states since California’s Mount Lassen had blown in 1917.

For two months, St. Helens shook with 10,000 tremors as scientists and sightseers converged on the awakening giant.

Symbolizing, perhaps, the human resistance to change was Harry Truman, the crusty, 83-year-old proprietor of the Spirit Lake Lodge. He vowed not to move to safety.

But what any human did was irrelevant to the immense forces building inside the Earth. By May, St. Helens was bulging with magma, or molten rock. The cone that Lewis and Clark admired was bursting at the seams.

At 8:32 a.m. on May 18, the mountain shuddered with another quake and the weakened north flank simply gave way, creating the largest landslide in recorded history. Half a cubic mile of mountain surged down the Toutle River Valley.

Gassy, trapped magma instantly shot from the mountaintop in a 650 mph burst that killed just about anything within 12 miles. Thousands of acres of majestic trees lay sandblasted and scattered like toothpicks.

A purplish particle plume shot upward for nine hours, sending flourlike ash across Washington and Idaho and into Montana. Street lights clicked on at midday. Within two weeks, wisps of the ash had drifted around the globe.

Fifty-seven people, including Truman, died from the eruption, most asphyxiated by the ash. Many more would have been killed in nearby logging camps if the mountain had erupted on a weekday.

The landslide swamped part of the Toutle River Valley, a surge up to 600 feet deep wiping out about 200 homes. Sediment choked the Columbia River and disrupted shipping for three months.

Mount St. Helens lost more than 1,300 feet of elevation.

And while rescuers, engineers and scientists dug through the mess, the mountain staged five more ashy eruptions that year and built a gigantic lava dome in the eerie new crater. For the next six years the volcano, every so often, spewed ash and rocks. But Mount St. Helens quieted down.

For living things, the clock started ticking.

At the crater’s edge burbles Loowit Springs. There, in the 194-degree sludge, University of Washington microbiologist John Baross and his colleagues found primitive, heat-loving microscopic creatures.

These single-celled organisms, called archae, are thought to be the most ancient forms of life on the planet. Such creatures thrived in the highly volcanic Earth of more than 3 billion years ago and are thought to have evolved into what we are today.

Such microbes, known as archae, also are found in hot deep-sea vents.

“There’s pretty strong evidence that these environments are our origin,” said Jim Holden, a University of Washington co-researcher with Baross. “Mount St. Helens is a window to what our planet was like.”

It took more than a decade to make Mount St. Helens accessible to the casual visitor.

The first fear was the danger of renewed eruptions, but seismic monitors now can warn of such hazards. In fact, knowledge gained from St. Helens resulted in early volcano warnings in the Philippines and elsewhere, saving tens of thousands of lives.

Some St. Helens visitors found it a challenge to satisfy their curiosity. A snaking road on the east side offered stunning views of the crater, but getting there from Seattle took nearly four hours, and the road was closed in winter.

Many made it only as far as the Mount St. Helens Visitor Center, five miles east of Interstate 5, which offers a distant view of the crater from 26 miles away. The center does offer exhibits and films depicting the May 18 events.

Two years ago, a second major visitor center opened farther up the highway, at Coldwater Ridge. There, the displays are high-tech - featuring video walls and interactive screens - and the outdoor trail takes tourists into an intense volcanic landscape.

By late next year, Highway 504 will reach still another seven miles to a new public “observatory” at Johnston Ridge.

Towns ringing the mountain are staging their own anniversary bashes. The village of Cougar, for example, today begins Big Blast Days, featuring everything from a Hot Lava Throwing Contest to a visit from Sasquatch.

Two new visitor centers on state Route 504 hold grand openings today:

The Hoffstadt Bluffs Visitor Center, a Cowlitz County project, offers a gift shop, helicopter tours and an information booth.

Six miles farther east, the Mount St. Helens Forest Learning Center, operated by Weyerhaeuser, shows the eruption’s effect on the forest and how it was replanted.

As the public gains more access to the mountain, Northwesterners will join hordes of foreign tourists in discovering that volcanoes played a significant part in shaping the region’s geography and character.

With this access, said Ecologist Peter Frenzen, who coordinates research at the monument, comes a responsibility to stay on the trails and to let the fragile ecology follow its own path of development.

A lupine plant struggling in the pumice might not survive bootsteps.

“Like the time traveler of science fiction,” he said, “the monument visitor has the potential to change history forever.”