Black Pool is in the West Thumb Geyser Basin of Yellowstone National Park, Wyoming. The pool reflects a sky with billowing clouds. Microbes adapted to near-boiling temperatures tint the pool’s water dark green and paint the edge bright orange. Yellowstone Lake is in the background.
Yellowstone’s geothermal “plumbing” is constantly rearranging itself. That accounts for the puzzling names of some pools and geysers. In the summer of 1991, a few months after I took this picture, an increased flow of hot water raised this pool’s temperature and temporarily turned it into a small geyser. It was too much even for the heat-loving bacteria. Thanks to this “steam cleaning,” Black Pool is now bright blue.