Yellowstone National Park's Morning Glory thermal spring used to be deep blue but its current yellow and green hues were caused in part by tourists throwing coins, rocks, and assorted crap into the pool.
The detritus has the effect of "partially blocking the underground heat source and lowering the temperature of the spring to a range habitable by photosynthetic microorganisms that probably didn't live there before" and produce pigments that result in the yellow and green color, according to a Science Friday article.
The image at right is from 1966. Researchers from Montana State University have now created computer models that accurately simulate how the color changes, but the simulation has other applications as well. From Science Friday:
Using this initial model as a starting point, the authors hope to pursue collaborations with biologists to develop tools that incorporate chemical and optical data to monitor the microbial composition of pools like Morning Glory from afar, thus eliminating the need for costly on-site sampling. Indeed, for years, biologists from around the world have flocked to Yellowstone, where a new discovery could reveal a bacterium with pharmaceutical applications, an enzyme for renewable energy, or even shed light on the origins of life on our planet.
Top photo: Joseph Shaw, Montana State University