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Any place along the rifts on the earths crust have volcanoes that are hazard.

But Yellowstone is on a hot spot. Not to say there are not such around the world, but few are as studied as Yellowstone.

Last I knew, they figured that there is not to much eruptable magma under there. It needs to be around a certain per cent to be considered a very high risk.

Take Tambora for example. That one blew several thousand feet off the top. Now that it happened, the next eruption many believe wont be much. Not so with the likes of the Long Valley Caldera. That one has a whole lot of magma under it and it. Way more than they thought before.
A caldera forms when several vents open up round the rim. They join up by cracks and after a lot of the magma is let out, the whole center collapses because the support is gone holding up the roof..so to speak.

Watch the movie Volcano..very accurate on how a super eruption happens.

I was in the Yellowstone Park most of today (7/16). My wife and I entered from West Yellowstone and traveled to Norris Geyser basin. We walked the south trail past Steamboat Geyser, Puff n Stuff, Echinus and other geysers. On finishing the loop we were too tired to trek north into the Porcelain Geyser field. It's pretty, but the climb back up is pretty steep. Steamboat had no water flow at all, just a few wisps of steam. The hillside was dry.

We continued east to Canyon, then turned south along the Yellowstone River. Lots of bison out today. As we reached the Lake, we headed west, then north toward Old Faithful. We didn't stop since we visited just 2 weeks ago.

From an on the ground perspective, it was just drier than usual. Many springs that were wet and active in prior visits over the last 20 years were just dry and dead now. In past years, there were abundant herds of elk on the inbound road from West Yellowstone to Madison Junction. Today, a single bison along that road. Some Canadian geese on the river banks and sand berms. Zero elk.

We didn't stop in town upon reaching West Yellowstone. Too many people on the streets.