The term "Rainbow Pool" is most commonly associated with the Grand Prismatic Spring in Yellowstone National Park, though it is also a nickname for the nearby Morning Glory Pool. These pools are named for their vivid, multi-colored rings that mimic the spectrum of a rainbow. The colors are not caused by chemicals, but by thermophilic (heat-loving) bacteria and algae that thrive in the mineral-rich, hot water. The center of the pool is a deep, clear blue because it is so hot that nothing can live there; the blue is simply the result of water molecules scattering sunlight. As the water moves outward and cools, different species of bacteria take hold. The green ring is created by cyanobacteria, followed by yellow and orange rings created by bacteria containing carotenoids (the same pigment found in carrots). Finally, the outer edges often appear reddish-brown. The Morning Glory Pool was originally a pure "morning glory flower" blue, but it became more "rainbow-like" over the decades as tourists threw coins and trash into the vent, slightly cooling the water and allowing yellow and orange bacteria to migrate toward the center.