This is one of the geysers (Black Pool)at West Thumb Geyser area in Yellowstone National Park. Not long ago, Black Pool really was black. Lower water temperature allowed thick mats of dark green and brown thermophiles (heat-loving microorganisms) to grow in the pool, causing it to appear black. The water temperature rose during the summer of 1991, killing these organisms. Black Pool also erupted that summer and several times the following winter. Like Abyss, Black Pool is now one of Yellowstone's most beautiful pools.