Where is the world's biggest active volcano? Under Yellowstone Park in Wyoming. I had no idea. National Geographic's August 2009 issue offers some staggering statistics and eye-popping graphics. The biggest eruption of Yellowstone (2.1 MYA) ejected enough material to bury the entire state of California under 20 feet of debris. And consider this:

The last three super-eruptions have been in Yellowstone itself. The most recent, 640,000 years ago, was a thousand times the size of the Mount St. Helens eruption in 1980, which killed 57 people in Washington. But numbers do not capture the full scope of the mayhem. Scientists calculate that the pillar of ash from the Yellowstone explosion rose some 100,000 feet, leaving a layer of debris across the West all the way to the Gulf of Mexico. Pyroclastic flows—dense, lethal fogs of ash, rocks, and gas, superheated to 1,470 degrees Fahrenheit—rolled across the landscape in towering gray clouds. The clouds filled entire valleys with hundreds of feet of material so hot and heavy that it welded itself like asphalt across the once verdant landscape. And this wasn't even Yellowstone's most violent moment. An eruption 2.1 million years ago was more than twice as strong, leaving a hole in the ground the size of Rhode Island.

The big question for most of us is when and when this monster will once again explode. We're not sure:

The odds of a full, caldera-forming eruption—a cataclysm that could kill untold thousands of people and plunge the Earth into a volcanic winter—are anyone's guess; it could happen in our lifetimes, or 100,000 years or more from now, or perhaps never.