Located in the geothermal areas of Yellowstone National Park, only three miles above the Earth’s molten rock, are gurgling, sulfurous pools of viscous boiling mud. These mud pots are a type of hot spring formed when water is in short supply. What seems to happen is that the smelly hydrogen sulﬁde present in hot springs creates sulfuric acid, which eats away at the surrounding rock and turns it to clay. When the clay gets thick enough, the water source gets clogged and, eventually, the spring becomes a mud pot. These curious pools contain clay, volcanic ash, and other mineral matter that can give them unusual colors; one very well known group of mud pots in Yellowstone, arranged along the Artist Paint Pot Trail, includes reddish, yellow, blue, and green pools. Often the boiling mud squirts over the edge of the pool, resulting in a mud volcano, which can rise to a height of three to ﬁve feet.