Several million people come to Yellowstone National Park every year, looking in wonder at grizzlies and geysers, majestic wildlife poised like living sculptures on hillsides, giant soaring birds that might swoop above or below in the mountainous terrain.
It all rests on what could be called a slow ticking time bomb -- one that might take hundreds of thousands of years to blow, but will eventually.
Yellowstone is actually the world's largest mega-volcano measuring at 53 miles across, and it has produced the planet's most monstrous eruptions.
In the largest explosion in all of earth's history, it left a crater, also known as a caldera, so large you could drop Tokyo in the middle of it and have room left over.
Yellowstone geologist Hank Heasler said the explosion blasted 600 cubic miles of molten rock out of the ground and shot up enough ash to bury New York City under a mile of the debris.
"That's 6,000 times larger than Mount St. Helens," he explained.
What it would look like if it happened again? Pyroclastic blasts would annihilate all of Yellowstone's natural beauty, any wildlife that hadn't fled, any humans unfortunate enough to be within dozens of miles. Ash clouds would spread across much of the U.S.
A Geological Society of London report revealed that so much sulphuric acid would be created in the atmosphere, it would block sunlight and plunge earth's temperature anywhere from 9 to 15 degrees for years.
That report stated, "Such events could result in the ruin of world agriculture, severe disruption of food supplies, and mass starvation. The effects could be sufficiently severe to threaten the fabric of civilization." It could lead to possibly one billion casualties.
The same report predicted the effects of another super-eruption would be like an asteroid half-a-mile wide hitting the earth.
But will it happen in Yellowstone again?
Heasler said since the mega-eruptions, the caldera's been filled in by some 80 smaller eruptions that spewed up lava. It's inevitable more will come.
"We know those are in Yellowstone's future," he said. "But the big, super volcanoes, we do not know."
Most scientists say tourists shouldn't worry about visiting Yellowstone. Signs of a coming catastrophic eruption would be clear weeks, maybe even decades or centuries before the big blow.
Yet even without a mega-eruption, there's still menace in the park's borders. The bison may look docile, but actually hurt more people every year than the bears do.
Yellowstone geologist Cheryl Jaworowski says there's a dangerous beauty to the park's geothermal features -- it is shooting geysers and pools of bubbling water.
"Some of them are very acidic like battery acid, and it can be hot battery acid," Jaworowski explained.
"There have been five times as many people that have died from interacting with the thermal features than bears and about ten times as many people that have been injured," she added.
Half of the world's geysers are at Yellowstone-- the result of water heated by a huge magma chamber four to six miles below the surface, America's hottest hot spot.
It's what would cause the next mega-explosion and that threat is what keeps scientists locking GPS monitors on the park's surface 24 hours a day, and why they know the center of Yellowstone is rising about two inches every year.
Till then, Yellowstone is a wonder to behold, not a threat to fear.
*Originally published August 28, 2009