Thinking about the deep underground as pitch dark, void of light and oxygen makes a nazi concentration camp sound like heaven. Hollywood movies commonly depict underground life as ugly sanguinious creatures burrowing their way out of the earth to leach havoc on their human prey. However this is actually making mockery of the true inhabitants of the deep subsurface as the review by Amend and Teske shows (2005). According to human standards, life processes are maintained by a surface ecosytem run predominantly by the sun from which plants harvest energy and convert it into food which all life forms dependent on. In the course of this process called photosynthesis, oxygen which surface creatures breath is produced. So the ultimate source of food and oxygen for most surface creatures such as humans and all the wild life we know comes from the photosynthetic action of plants. This therefore explains why surface life will be annihilated if we do not take care of plant life on earth including the dissappearing rain forest of the Amazon and Africa. Now what happens deep within the earth? The past 3 decades have revealed that life is abundant in the deep underground from the hundreds of scientific articles that have been realised in this discipline. Mostly microorganisms including bacteria and a very strange class of organisms called archaea that can live in very extreme environments for example in environments hotter than boiling water or acidic enough to scar human tissue have been isolated at depths as deep as 3 kilometers underground and in deep hotsprings.
These organisms if exposed to oxygen die instantaneously. Such underground microorganisms have been estimated to contain as much carbon as all earthly plants combined and have contrived several mechanisms for survival. For example they eat rocks and metals and breath gases such as methane and hydrogen. They have developed extensive systems for coping in the high pressures existing underground that will crush a human flat, low nutrient levels and lack of oxygen and sun. In some systems the base of the food chain has been postulated to be organisms breathing hydrogen and carbon dioxide upwelling from within the eath's crust to produce methane and acetate which the rest of the extant community members depend on. Such systems have been identified in granitic rock of Sweden and Finland. With such knowledge, scientists are now beginning to postulate that extra-terrestrial life is possible in the deep underground of far away planets. See the reviews by Jan P. Amend and Andreas Teske (2005). Expanding frontiers in deep subsurface microbiology. PALAEO, 219, 131-155.