My vote is for the Yellowstone Supervolcano. If you take a look at the image showing potential range of the ash cloud, you can see it might cause a few problems if it were to blow. Maybe not a next-thirty-years type of event, but still an interesting possibility with potential for tremendous casualties, and very little that can be done in the way of prevention.

http://www.earthmountainview.com/yellowstone/yellowstone.htm