Yellowstone has the largest and most varied collection of hydrothermal features on Earth. Geysers, hot springs, mud pots and fumaroles bubble, gurgle, hiss and spout throughout the area. The presence of so many thermal features and earth tremors indicates that molten rock lies just beneath the Earth's surface.

Hydrothermal features are caused by water and molten rock in the Earth's crust. Much of Yellowstone's precipitation seeps into the ground via the porous volcanic rock. Some of the water stops at the water table, returning to the surface through cold springs. The rest, however, continues deeper. The water seeps down several thousand feet, where it is heated by molten rock to temperatures far in excess of the boiling point, perhaps more than 400°F. Because of the tremendous pressure, it cannot boil or turn to steam, so the water escapes through a "plumbing system" deep within the Earth. As it rises through the porous rock channels, its pressure goes down, and the water boils at last.

Fumaroles are Yellowstone's hottest surface features. Their underground channel systems reach down into the hot rock masses, but contain very little water. When water does drain into the fumarole's plumbing, it converts instantly to steam. Hot springs are closely related to geysers, but the difference is that the underground channel systems are not constricted. Water circulates to the surface where heat escapes through evaporation or runoff. The cooler water returns to the underground system, keeping it in equilibrium.