After our adventures on horseback we went back in to Yellow Stone to see the "Grand Prismatic Hot Spring." It is a circular hot spring the diameter of a football field. The water in the center is over 160 degrees (Fahrenheit) and there is almost nothing living in it. The water around the edges is cooler and the temperature gradient is visible to the naked eye due to colorful bacteria communities that form rings around the outside. Each ring consists of a mix of bacterial species that can exist at that temperature. The whole thing is like a biological thermometer. The exact color profile varies seasonally as the composition of bacteria varies.

Speaking of bizarre and colorful wonders, here is a much smaller one I saw on the path to the Grand Prismatic Spring overlook. I have not identified this one.