If a person were asked to name the element most important to sustaining life, chances are he or she would say oxygen. It is true that many living things depend on oxygen to survive, but, in fact, carbon is even more fundamental to the sustenance of life. Indeed, in a very real sense, carbon is life, since every living thing contains carbon and the term organic refers to certain varieties present in all life-forms. Yet carbon, in the form of such oxides as carbon dioxide as well as carbonates like calcium carbonate, is a vital part of the inorganic realm as well. Hence, the carbon cycle, by which the element is circulated through the biosphere, geosphere, atmosphere, and hydrosphere, is among the most complex of biogeochemical cycles.