A mud pot is a natural double boiler! Surface water collects in a shallow, impermeable (usually due to a lining of clay) depression that has no direct connection to an underground water flow. Thermal water beneath the depression causes steam to rise through the ground, heating the collected surface water. Hydrogen sulfide gas is usually present, giving mud pots their characteristic odor of rotten eggs. Some microorganisms use the hydrogen sulfide for energy. The microbes help convert the gas to sulfuric acid, which breaks down rock into clay. The result is a gooey mix through which gases gurgle and bubble.
After coming upon Mud Volcano during his 1871 expedition to Yellowstone, Ferdinand Hayden described the mudpot as "the greatest marvel we have met with."
The sights, sounds, and smells of areas like Artist and Fountain paint pots and Mud Volcano make these curious features some of the most memorable in the park.
Minerals tint the mudpots with such a large palette of colors that the mudpots are sometimes called "paint pots." Iron oxides cause the pinks, beiges, and grays of the Fountain Paint Pots (virtual tour).