I can't possibly explain how cool that Yellowstone National Park is. It's a place that you have to see for yourself and when you leave, you take away a great appreciation for just how big, diverse, amazing and awesome our country is.
I think people go there for 2 reasons: the wildlife and the geothermal activities. Basically there is a volcano really far below the park that causes geysers, paint pots and hot springs all over the place. Half of the geothermal activities on Earth occur there. I was only there a few hours but it's definitely in my plans to go back.
Check it out: (these pictures are courtesy of my flying partner, Denise, who organized the rental car and trip and actually had her camera with her)