They eat hydrogen, breathe carbon dioxide, and belch methane. And they form the root of an ecosystem unlike any previously known on Earth. Meet the methanogen, a tiny organism living in complete darkness 660 feet (200 meters) underneath the surface of Idaho. Researchers report in the Jan. 17 issue of the journal Nature the discovery of a community of various organisms dominated and supported by these methanogens, creatures they say could represent just the sort of life to look for when turning over rocks on Mars. The work, along with another report this week of life found in extreme conditions in Antarctica, adds to mounting evidence for life’s tenacity and creativity, fueling increased speculation about the prospects for life on other worlds.