Mud volcanoes, also called mud domes, are small volcano-shaped cones of mud. Like mud pots, mud volcanoes have little water. Their dominant fluid is gas. As the gas rises through the ground it dissolves the rock and makes mud just as a mud pot would. But in the case of a mud volcano, the hot mud does more than boil and churn. It flows like lava or explodes into the air. Mud volcanoes erupt when the pressure from underground gas is more than the earth can bear. And while the geothermal feature at Yellowstone called Mud Volcano not longer flows it once erupt enough mud to cover the surrounding trees.