Yellowstone

Delanceyplace.com has a selection from A Short History of Nearly Everything: Special Illustrated Edition by Bill Bryson, with tidbits for your next Yellowstone National Park vacation: 

In the 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geological Survey became puzzled about something: he couldn't find the park's volcano.By coincidence, just at this time, NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone, copies of which some thoughtful official passed on to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers. As soon as Christiansen saw the photos, he realized why he had failed to spot the volcano: virtually the whole park, over two million acres, was a volcano. The explosion had left a crater more than forty miles across, much too huge to be perceived from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
Yellowstone, it turns out, is a supervolcano. It sits on top of an enormous hot spot, a reservoir of molten rock that rises from at least a hundred miles down in the Earth. The heat from the hot spot is what powers all of Yellowstone's vents, geysers, hot springs, and popping mud pots. Imagine a pile of TNT about the size of Rhode Island and reaching eight miles into the sky to about the height of the highest cirrus clouds, and you have some idea of what visitors to Yellowstone are shuffling around on top of.Since its first known eruption sixteen million years ago, the Yellowstone volcano has blown up about a hundred times, but the most recent three eruptions are the ones that get written about. The last eruption was a thousand times greater than that of Mount St. Helens; the one before that was nearly three hundred times bigger and the one before that was at least twenty-five hundred times greater than St. Helens.The Yellowstone eruption of two million years ago put out enough ash to bury New York State to a depth of sixty-seven feet, or California to a depth of twenty. All of this was hypothetically interesting until 1973, when geologists did a survey and discovered that a large area of the park had developed an ominous bulge. The geologists realized that only one thing could cause this: a restless magma chamber. Yellowstone wasn't the site of an ancient supervolcano; it was the site of an active one. It was also at about this time that they were able to work out that the cycle of Yellowstone's eruptions averaged one massive blow every six hundred thousand years. The last one interestingly enough was just over six hundred thousand years ago. Yellowstone, it appears, is due.

Rico says that, when it goes, we're all screwed... (Remember the dinosaurs? Yeah, like that.)