Extremophiles are also remarkable for their ability to find nourishment within their barren, rocky habitats. The most basic needs of all life forms are carbon and energy. We obtain these basics from sugars, fats, and other organic compounds in the plant- and animal-based foods that we eat. Some extremophiles also live off of dead plants in a way: They utilize ancient buried plant life, often in the form of oil, coal, or other hydrocarbons, as a food source.

But it turns out that there are other sources of organic carbon buried deep within the Earth that did not originate from the photosynthetic process of plants. Long ago when our planet was first forming, many of the meteors that bombarded the primitive Earth were a type known as carbonaceous chondrites. These contain organic forms of carbon (molecules with both carbon and hydrogen atoms), as well as nitrogen and sometimes traces of water. Although collisions between the Earth and these types of meteorites are rare today, a recent impact occurred near the town of Murchison, Australia, on September 28, 1969. The Murchison meteorite was analyzed and found to contain not only organic carbon but several amino acids. Carbonaceous chondrites like the Murchison meteor may have been important in the very origin of subterranean life three and a half billion years ago, in addition to being a source of nourishment for some extremophiles today.

The most amazing of the extremophiles are the lithotrophs, or "rock-eaters," which live off of the rock itself. They obtain their carbon from carbon dioxide gas in a process that is similar to photosynthesis in some ways. However, unlike plants, lithotrophs are in the dark, so they must find an alternative to solar energy to power the uptake of carbon (and to power other life functions). It is only recently that we have discovered that the lithotrophs derive their energy by stripping off electrons from the atoms of inorganic minerals in the surrounding rocks, or from hydrogen atoms of hydrogen gas in the environment. This is a unique and amazing talent, one that allows lithotrophs to survive completely independent of the sun, organic food sources, and surface life.

One of the best studied of the subsurface lithotrophic microbial ecosystems (called SLiMEs, for short) was found in the early 1990s in the Columbia River Basin in the northwestern United States. This bizarre microbial community is embedded in a crystalline basalt rock aquifer thousands of feet below the surface. The anaerobic lithotrophs at the base of the dark food chain obtain their carbon from carbon dioxide gas, and laboratory studies suggest that they are fueled by the hydrogen gas produced when ancient water in the pore spaces reacts with iron-silicate compounds in the rock. They are methanogens that produce methane (natural gas) as a by-product of their metabolism, just as we exhale carbon dioxide.

The discovery of the Columbia River SLiME has global implications because we know that much of the Earth's continental crust is composed of similar basalt rock. If it is verified that hydrogen production from basalt-water reactions can provide enough energy to support an entire community of microbes, this will indicate a widespread "terrestrial energy" source (an alternative to solar energy) for subsurface ecosystems. Even if it turns out that these ecosystems depend on some trickle-down of nutrients from the surface, the fact that this common type of underground rock environment is populated supports the notion that the subsurface biomass of Earth is very large indeed.

The recent discoveries of Earth's subsurface microbes, with their exotic metabolisms, have caused the U.S. National Aeronautics and Space Administration (NASA) to overhaul completely its plans for future space missions. The lithotrophic ecosystems provide a model for the existence of contemporary life on Mars because basalt rock, liquid water, and bicarbonate (dissolved carbon dioxide gas) are believed to be present within the Martian subsurface. And we know from physical evidence collected by the Viking Landers that Martian soils contain abundant sulfur and ferric iron compounds, possible substitutes for oxygen in respiration.

Recently, procedures developed to study life in deep Earth proved very useful in evaluating a rare Martian meteorite found in Antarctica for signs of ancient life. Initially, researchers were almost certain that microscopic wormlike indentations (figure 2.3) were fossils of subsurface Martian bacteria. Soil scientists and microbiologists eventually concluded, however, that these were too small and probably were caused by chemical crystals of some sort, not living organisms. Most tests for signs of life in the rock have not turned up positive, but some feel that small grains of a mineral called magnetite contained in the rock are similar to grains produced by certain subterranean Earth microbes.

Although the evaluation of the Martian meteorite may never be conclusive, many scientists now believe that microbial life on other planets is a real possibility. It's exciting to think about. Clear evidence of extinct microbial life on another planet would be enough to both thrill us and throw us into an existential dither. But in our space explorations to date, we have been looking for life in all the wrong places. Our expensive and sophisticated space probes have literally just been scratching the surface, when the real action may be below, perhaps miles below, the surface.

NASA has recently established the National Astrobiology Institute, both to study life in extreme habitats here on Earth and to plan the details of the search for life in future space missions. The interests of scientists from different disciplines - geology, astronomy, soil microbiology, evolution - are merging around the study of both extreme habitats on Earth and the potential for life beyond our planet. As a result, the next couple of decades are likely to be filled with many exciting discoveries of life in the underground, and perhaps elsewhere.