Grand Prismatic Spring in Yellowstone National Park is one of the better known thermal features there. The brilliant blue center turns green then yellow and several shades of orange along the edges. The center blue is because the water is deep, clear, and hot, too hot for anything to grow in it. The water absorbs wavelengths of light in the red end of the spectrum leaving the blue light to be reflected back. Towards the edges, where the water is cooler, heat loving bacteria grow, giving the characteristic oranges. Different bacteria which are different colors thrive at different temperatures.
The best place to see the spring is not the Midway Geyser Basin parking area, that gives you a view too close to the spring to appreciate the full effect. Instead, you want to take the trail that starts at the Fairy Falls parking area and goes to Grand Prismatic Spring overlook.
