Answer: Trash

Visitors have flocked to see the Morning Glory thermal spring in Yellowstone National park for decades to enjoy its vibrant colors. Among the mud, rock, and simple pine forest backdrop, the spring is a burst of intense color: deep oranges, bright yellow, and emerald greens.

The current coloration of the spring is actually a rather recent change, however, and if you went to visit the spring before the 1950s, you’d see something entirely different. Back then, the spring was an intense and beautiful blue color (a shade not seen at all by modern visitors). In fact, the very name of the spring, Morning Glory, is a reference to the intense blue color of Morning Glory flowers.

So what happened to the bright blue color? The color of the spring is the result of microbial activity. There are thousands of different microbes active in hot springs and they all have rather specific temperature preferences. Years ago, the temperature of the spring was higher and the blue pigment producing microbes flourished. Today the temperature is slightly lower and the microbes that prefer the new temperature range produce yellow-orange and green pigments.

If visitors to the park are disappointed that the spring is no longer blue, they ultimately have no one to blame but themselves. Decades of tourists tossing coins, rocks, and other debris into the spring changed the flow of hot water underground and lowered the temperature of the surface water. The blue-pigment microbes died out and the new microbes took over. Unless some subterranean shift occurs where more hot water can again flow to the surface, Morning Glory will never be blue again.