| A hot spring is a spring produced by the emergence of geothermally heated groundwater that rises from the Earth's crust. Their water may be lukewarm or boiling hot. Especially when the water is hot, they tend to harbor thermophilic bacteria with sometimes spectacularly colorful displays. |
 |
 |
Haukadalur, Iceland; 16 & 23 July 2014. The blue color of the water results from colloidal silica. |
 |
 |
 |
 |
Yellowstone (Wyoming), USA; 22 August 2017. Dragon's mouth spring and fumarole. |
 |
 |
Yellowstone (Wyoming), USA; 16 August 2017. Emerald pool (left) and Opal pool (right). |
 |
 |
 |
 |
 |
 |
Yellowstone (Wyoming), USA; 16 & 18 August 2017. Grand prismatic spring. |
 |
 |
Yellowstone (Wyoming), USA; 22 August 2017. Unnamed small spring. |
 |
 |
 |
 |
Yellowstone (Wyoming), USA; 22 August 2017. Morning glory pool. |
 |
 |
 |
Yellowstone (Wyoming), USA; 16 & 18 August 2017. Sapphire pool. |
 |
 |
 |
 |
Yellowstone (Wyoming), USA; 22 August 2017. Surprise pool. |