On the walk from Upper Geyser Basin to Midway Geyser Basin, the trail takes you past Morning Glory Pool. This is a large hot spring that is a great example of how different water temperatures cause the different colors in the water. The blue water at the center, down towards the bottom of the pool, is the hottest. The more shallow yellow water, towards the edges of the pool, has a lower temperature.
This pool, however, is also a great example of how ignorance can permanently alter our landscape. Because people throw things into the pool, the vent gets clogged and the water temperature decreases in the pool. This results in less of the vivid blue, and more of the yellows and browns. The park service has actually "vacuumed" this pool's vent in an effort to keep it clear of the stuff humanity throws into it.
I never would have imagined that people would think it was okay to throw their trash into something as beautiful as Morning Glory Pool. Sheesh.