Somebody estimated that 90 percent of the Earth's biomass lives underground, in the form of bacteria. Pound for pound, nine times more life lives in the dirt than up here where we typically think of life as living. Bacteria have been found in oil resevoirs four miles below the Earth's surface. You pick the most inhospitable places you can imagine--the 650 degree waters boiling out of vents at the bottom of the ocean--265 times more pressure than what we live with--and there are bacteria there.
This picture shows you what can happen when a bacteria that thrives in our armpits and crotches everyday gets under the skin and settles a colony.
Here's a nice S. J. Gould article on the ubiquity of bacteria.
category: that which cannot be named