Hot springs, mud pots, fumaroles and geysers are four types of hydrothermal features. All four are created by an explosive mix of heat, water and rock yet the combination differs for each. Both geysers and hot springs have lots of water. It runs freely in hot springs but not in geysers. A geyser's water flow is constricted by rock. Fumaroles are the hottest of the four hydrothermal features. The water in fumaroles turns to steam before reaching the surface. Mud pots have little or no water at all. They have lots of gas instead. The gas, as acidic as stomach fluid, dissolves the rock creating pools of mud.