While many visitors to Yellowstone may be oblivious to the science at work, geologists and volcano experts have long known about the region's explosive prehistoric past.

An explorer identified the massive caldera in 1871. But satellite-based global positioning systems (GPS), gravity mapping and a seismic network are all helping scientists isolate more details of the area. And other new information is being uncovered all the time.

In fact, the Yellowstone caldera is the place where Smith and other geophysicists are beginning to finally pull aside the curtain that's been hiding one of geology's most stubborn secrets: the strange workings of Earth's "hot spots."

These "hot spots" are areas of volcanic activity not found in the usual location, at the edges of Earth's tectonic plates. Why they exist is a subject of scientific debate. And there are many around the world, not just Yellowstone.

But at this hot spot's current position under Yellowstone there have been three massive eruptions: 2.1 million, 1.3 million and 640,000 years ago. While those eruptions have been spaced roughly 800,000 and 660,000 years apart, three events are not enough statistically to declare this an eruption pattern, explains Smith.

Though Yellowstone could erupt again someday, there is no evidence that the caldera is readying for another massive blast, says Smith. That outlook is shared by Jake Lowenstern, the U.S. Geological Survey's lead geologist at the Yellowstone Volcano Observatory.

Volcanologists with the U.S. Geological Survey believe that supervolcanoes are likely to give decades — even centuries — of warning signs before they erupt. The scientists think those signs would include lots of earthquakes, massive bulging of the land, an increase in small eruptions, "swarms" of earthquakes in specific areas, changes in the chemical composition of lavas from smaller eruptions, changes in gasses escaping the ground and, possibly, large-scale cracking of the land.

None of those indicators are present at Yellowstone, says Smith.

There is no argument that a major eruption at Yellowstone in modern times would be devastating. It would obliterate the national park and nearby communities, spread ground-glass-like volcanic ash from the Pacific coast to the Midwest, and cause worldwide weather changes from the airborne dust and gases, according to Smith, who described the potential effects in detail in his book Windows Into the Earth, published in 2000.

A modern full-force Yellowstone eruption could kill millions, directly and indirectly, and would make every volcano in recorded human history look minor by comparison. Fortunately, "super-eruptions" from supervolcanoes have occurred on a geologic time scale so vast that a study by the Geological Society of London declared an eruption on the magnitude of Yellowstone's biggest (the Huckleberry Ridge eruption 2.1 million years ago) occurs somewhere on the planet only about once every million years.

But there are several levels of eruptions smaller than Huckleberry Ridge and yet still much larger and more destructive than any volcano ever witnessed by modern man.

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