Norris shows evidence of having had thermal features for at least 115,000 years. The features in the basin change daily, with frequent disturbances from seismic activity and water fluctuations. The vast majority of the waters at Norris are acidic, including acid geysers which are very rare. Steamboat Geyser, the tallest geyser in the world (300 to 400 feet) and Echinus Geyser (pH 3.5 or so) are the most popular features."

• http://www.nps.gov/yell/planyourvisit/nnorris.htm