We now know that there are a lot of microbes living deep within the Earth, many of which have nothing at all to do with the organic world. They eat rocks or, rather, the stuff that's in rocks—iron, sulfur, manganese, and so on. And they breathe odd things too—iron, chromium, cobalt, even uranium. Such processes may be instrumental in concentrating gold, copper, and other precious metals, and possibly deposits of oil and natural gas. It has even been suggested that their tireless nibblings created the Earth's crust.
Some scientists now think that there could be as much as 100 trillion tons of bacteria living beneath our feet in what are known as subsurface lithoautotrophic microbial ecosystems— SLiME for short. Thomas Gold of Cornell has estimated that if you took all the bacteria out of the Earth's interior and dumped it on the surface, it would cover the planet to a depth of five feet. If the estimates are correct, there could be more life under the Earth than on top of it.
At depth microbes shrink in size and become extremely sluggish. The liveliest of them may divide no more than once a century, some no more than perhaps once in five hundred years. As the Economist has put it: "The key to long life, it seems, is not to do too much." When things are really tough, bacteria are prepared to shut down all systems and wait for better times.