Black Pool, a hot spring in Yellowstone National Park’s West Thumb Geyser Basin in Wyoming, was once cool enough for dark orange-brown cyanobacteria to grow across it. Blended with the water’s blue color, that growth made the pool appear very dark green, almost black, giving it its name. In 1991, shifting thermal activity redirected heat to Black Pool and nearby Abyss Pool, raising Black Pool’s temperature enough to kill the cyanobacteria and turn the water a vivid teal blue. On August 15, 1991, it also had frequent boiling eruptions. Its old name and new color highlight how quickly Yellowstone’s thermal features can change.