Black Pool is a hot spring in the West Thumb Geyser Basin of Yellowstone National Park. Until 1991 the pool was cool enough to allow the dark orange-brown cyanobacteria to grow throughout and combine with the blue of the water to make it appear dark green to almost black. Hence, the name "Black Pool." In 1991 shifting thermal energy to Black Pool and the nearby Abyss Pool occurred, causing them to heat up. The temperature of the Black Pool became hot enough to kill all the cyanobacteria and turn the pool a rich teal blue color. The pool remains extremely hot and even though it's one of Yellowstone's most beautiful and intense blue pools the name of the pool remains "Black Pool." West Thumb Geyser Basin Trail is a heavily trafficked 1-mile loop trail located near Yellowstone National Park. The path is suitable for all skill levels and it's primarily used for hiking, walking, nature trips, and bird watching from May until October.