What can microbiologists who study human bowels learn from those who study the bowels of Earth? Jillian Banfield trades in hell holes. In September, she could be found wading through the dark, hot, sulphurous innards of Richmond Mine at Iron Mountain, California, where blue stalactites ooze the most acidic water ever discovered, with a pH of −3.6. A year before that, she was pumping up a toxic soup of uranium, arsenic, molybdenum and other metals from underneath a decommissioned nuclear-processing site in Rifle, Colorado. From both sites she took samples back to her lab at the University of California, Berkeley,...