A fire in a forest, unsurprisingly. A forest fire tends to be more environmentally damaging than most other fires (barring stuff like chemical fires, etc.) because they can destroy a whole hell of a lot of trees, plants, animals, structures, and people very quickly. Of course, forest fires can serve a beneficial purpose -- small forest fires are sometimes nature's way of cleaning up all the dead leaves and fallen trees that litter the ground. Without a few small forest fires to get rid of the dry kindling on the ground, a single fire could burn out of control as it eats up all the fuel on the forest floor.

Many forest fires have natural causes, such as lightning strikes or badgers playing with matches, but many are also caused by people -- some campers don't make sure their campfires are properly extinguished, some people drop their cigarettes in the wrong places, and some folks are just mean-ass arsonists.

Nowadays, it seems like there's a major drought every year, so the forests are really, really dry. Massive forest fires are getting to be a regular occurence, so watch yourself when you go camping, 'kay? Like Smokey says: Only you can prevent forest fires.