Black Pool
Encyclopedia
Black Pool is a hot spring
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

 in the West Thumb Geyser Basin of Yellowstone National Park
Yellowstone National Park
Yellowstone National Park, established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872, is a national park located primarily in the U.S. state of Wyoming, although it also extends into Montana and Idaho...

 in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

.

The pool was cool enough up until 1991 for dark orange-brown cyanobacteria to grow throughout the pool. When combined with the blue of the water, the pool appeared to be an exceptionally dark green to almost black, hence the name

An exchange of function took place in 1991, shifting thermal energy to Black Pool and nearby Abyss Pool
Abyss Pool
Abyss Pool is a hot spring in the West Thumb Geyser Basin of Yellowstone National Park in the United States.- History :The pool was named by Chief Park Naturalist Clyde M. Bauer, possibly after a reference to Lieutenant G.C. Doane's 1870 description of a spring in this area which spoke of the...

, causing them to heat up. Black Pool's temperature became hot enough to kill all the cyanobacteria in the pool, turning the pool a rich teal blue color. The pool also had frequent boiling eruptions on August 15, 1991, doming the water to 3 feet and causing heavy runoff. Black Pool remains extremely hot, and is now one of Yellowstone's most beautiful and intensely blue pools. The name of the pool remains "Black Pool."
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