Uncontrollable flames that are fueled by the dry weather, oxygen, and all the dry underbrush. Fuel, oxygen, and a heat source is what firefighters refer to as "The Fire Triangle". Only four out of five wildfires are caused by humans, but nature is also to blame because it helps fan the flames with oxygen.
Yellowstone's "Summer of Fire"
In Yellowstone national park, the summer of 1988. It was one of the worst wildfire there. It burnt more than two million acres of land, almost a third of the park. Luckily no lives were lost, and up to 25,000 firefighters were dispatched to help take care of the blazes. Because of Yellowstone's popularity and because of the size of the fire, it was soon covered by the media. Since then Yellowstone has made a good recovery.
I got this information from http://www.mnn.com/earth-matters/wilderness-resources/stories/10-of-the-worst-wildfires-in-us-history