Yellowstone has an estimated 10,000 thermal features, from which only the three percent are geysers. The rest are hissing fumaroles, bubbling mud pots, steaming pools, or warm seeps. Geysers at Yellowstone are generally small, sputter, splash, and they barely reach 10 feet in height. Nowadays, only six grand geysers that erupt 100 feet or higher exist. Old Faithful is the most famous geyser which erupts approximately every 45 to 90-minutes.

In Yellowstone, pools, hot springs, and run-off channels exhibit in all colors of the rainbow. Algae and bacteria are mainly responsible for making those brightly colored run-off channels—besides, the different temperatures of water permit and cause differences in plant intensities of color. For instance, the run-off channel from a hot spring is white and bright, because only some single-cell bacteria live in its boiling water, which reaches 199°F, while pure water boils at around 212°F at sea level.