The sizzling basin known as Sulphur Caldron and Mud Volcano is one of the most volatile hydrothermal areas in Yellowstone National Park. And it is one of the most eerily intriguing. Sulfur is responsible for the peculiar sights, sounds and smells. - Iron sulfide paints mudpots and fumaroles shades of brown and gray. Hydrogen sulfide gurgles and hisses and produces a pungent rotten egg smell. Sulfuric acid, twice as acidic as battery acid, cooks the terrain creating a graveyard of skeleton trees. - Sour Creek Dome, a volcanic vent called a resurgent dome, is the source of instability. It's of l... Read Moreittle consequence that Mud Volcano, the hydrothermal feature for which the area is named, no longer throws mud nor rumbles noisily.