A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. The hot spring pools contain microorganisms known as thermophiles, a type of extremophile. They are organisms that exist in the most unlikely, unlivable, and extreme places, hence their name.
The color comes from the way that these thermophiles reflect the light of the sun. There are geothermal hot springs in many locations all over the crust of the earth, but many of these wonderful water bodies are concentrated in Yellowstone National Park, USA.