Morning Glory Pool, Upper Geyser Basin, Yellowstone National Park

These pools are very hot and dangerous, but it is the heat that determines the color. Different kinds of algae grow in the pools at different temperatures, as can be seen in this picture. The green, yellow and brown colors are all caused by different algae. This pool used to be brilliant blue, thus the name, but so many coins and so much other trash has been thrown in the pool over the years that the vent has been partially blocked and the temperature has changed and with it the color of the pool.