Much of Yellowstone National Park sits inside an ancient volcanic caldera that was formed by massive volcanic eruptions 640,000 years ago. 
For hundreds of thousands of years following that eruption, subsequent lava flows slowly filled in most of the caldera.
Even now, in some places, nearly molten rock resides as little as 2-5 miles below the surface.
Heat from the volcanic activity makes its presence known by heating ground water and creating the thermal features that are able to be seen while traveling through the park.
The four basic types of hydrothermal features found at Yellowstone Park are: geysers, hot springs, fumaroles, and mudpots.
There are a total of 10,000 hydrothermal features altogether found at Yellowstone Park.


The Norris Geyser Basin and Fountain Paint Pot areas are home to many hydrothermal features, which are fueled by an enormous magma chamber seated below much of the park.