A "caldera", is a flat volcano also known as a super volcano, and Yellowstone National Park has one that is two-thirds of the entire park. This will eventually blow it's just a question of time as to when it will happen. When it does blow this will be a catastrophic event. The pyroclastic flows will move like lightening ( this is an assumption,(that I believe) that it will be pyroclastic flows, since nobody in this day and age has ever seen such a cataclysmic event) across the land. Yellowstone's caldera is the largest one known, in the United States (there is another caldera in California and one in New Mexico that the scientists, volcanologist's, and geologists know of).
There are over 20 known calderas on this planet. Toba on( now a lake stands where the eruption blew called Lake Toba),one of Indonesia's (they have the most volcanoes) islands, was a super volcano. It's thought [by those who came to this conclusion], to have almost exterminated the human race, with only a few thousand left living after the mega eruption blast, almost 80,000 years ago.
Everything and everyone on this planet Earth, will be affected when this occurs. You know the scientists, volcanologist's, and geologists, have their thoughts as to when Yellowstone's caldera will blow. There is one that does know ...and the caldera of Yellowstone isn't telling anybody...yet.
If you'd like to check out more information about Yellowstone's caldera, calderas itself (better to put in your search engine "super volcanoes" instead of calderas),and Toba, you can use your search engine or do like I did...watch National Geographic, The Discovery Channel, and the History channel where the episodes are on throughout the year. Check the channel's sites itself to get the dates and times when it will run again or check your TV/Cable guide for the listings. The other sources I found several times several months ago, using various search engines, is listed below.
I hope you enjoyed this little bit of knowledge!
**UPDATE**
The first six months of 2009, Yellowstone had a swarm of hundreds of earthquakes, some with a magnitude in the 3-4 range. In a 10 day period, close to 500 earthquakes rumbled. Still, it's stated on the Yellowstone National Park website there is still no cause to be alarmed.
