(Top) Park visitors take advantage of a warm fall day to view an eruption of Cliff Geyser in the Black Sand Basin Group of Yellowstone's Upper Geyser Basin (take the online tour).

(Top left) Cliff Geyser was so named because of its ledge-like wall of geyserite that separates it from Iron Spring Creek.

(Above) Emerald Pool is one of Yellowstone's most famous hot springs. It was named in 1872 during an expedition led by geologist Ferdinand V. Hayden. The pool's color is caused by reflected blue light from the water combining with the yellow algae growing in its bowl.

Learn more about Yellowstone's hydrothermal features.