I recently watched a special on NOVA about Yellowstone National Park.  In it, the authors voiced a genuine concern over the swelling of the ground in the park and the increase of geothermal and seismic activity, gave the park's volcanic history, and predicted a catastrophic event happening in the not-so-distant future.  The Yellowstone caldera is due for an eruption, they stated, sometime in the next 200 years, and yes, it could be as destructive as Krakatoa was.   They mapped out an area encompassing four states that are in the danger zone.  Such an eruption would dramatically and permanently alter the western United States as we know it today.

I wish I could remember the name of the program, but you may want to check it out on their website.