Yellowstone National Park boasts dozens of geysers and broiling
eruptions. But they're nothing compared to the massive volcano
that bubbles beneath the park, and could unleash a world-
altering blast. Check out these images of the megablast-in-
waiting. National Geographic explains: Yellowstone is a volcano,
and not just any volcano. The oldest, most famous national park
in the United States sits squarely atop one of the biggest
volcanoes on Earth . . . 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 landsc