Steam rises from the Grand Prismatic Spring in Yellowstone's Midway Geyser Basin. Grand Prismatic is the largest hot spring in Yellowstone, and is considered to be the third largest in the world. The colors begin with a deep blue center followed by pale blue. Green algae forms beyond the shallow edge. Outside the scalloped rim a band of yellow fades into orange. Red then marks the outer border. Steam often shrouds the spring which reflects the brilliant colors.