Any disaster fiend will tell you that Yellowstone National Park is long overdue for a monster eruption that could leave as much as half the U.S. under a blanket of ash. And there are rumblings the big one could be imminent in the wake of a series of 30-plus mini-earthquakes in the park over the past few days—too weak to be felt by humans for the most part but picked up by the seismometers at the University of Utah.

After all, the geologic record shows that the giant caldera we affectionately call Yellowstone has blown every 600,000 years or so over the past 2 million years. The last big eruption? About 640,000 years ago when the park spit out about 240 cubic miles worth of rock, dirt, magma and other stuff.

But don't panic yet. Although the earthquake swarm continues, according to the Yellowstone Volcano Observatory, the volcano alert level remains normal. And a slew of larger earthquakes have occurred throughout the western U.S., Alaska, Puerto Rico and even Pennsylvania in the past week without incident, according to the U.S. Geological Survey.

In recent years, Yellowstone's caldera has been rising thanks to uplifting magma beneath it—leading to more cracks, hot springs and even more frequent eruptions of Steamboat Geysers. Paired with the earthquakes, such magma movement might presage an eruption—either big or small. Unfortunately, scientists can't really predict when the next such eruption will happen, and the range of possibilities is large: from later today to a million years from now.

How will we know if we should start worrying?  The real warning signs will be rapid changes in the shape of the ground as well as volcanic gases leaking from the ground, neither of which have been sighted—yet.

"Eruptions are far enough apart that there is a very low probability of the next eruption happening in our lifetimes or anytime soon," Daniel Dzurisin of the USGS told me in 2006. "The flipside is: [Yellowstone] has been active for millions of years and it's going to erupt again sometime."

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