Among all living organisms on planet Earth, those that are invisible to the naked eye are the most abundant, most widely distributed and are indispensable for the continuation of life which they themselves originated. This review article has been prepared by going through important historical as well as recent literature on microorganisms with the primary purpose of dissemination of knowledge. It starts with the discovery of microorganisms and takes the reader through the key developments that enabled us to understand the invaluable roles they play in our lives, their possible origin on this planet, their diverse ecosystems and the amazing recent findings of some their existence several kilometers below the Earth’s surface. Such subterranean communities appear to possess hitherto little known metabolic systems which are yet to be fully resolved. Moreover, whether these organisms originated in their ultra deep habitats or were transported down from the surface during the early formation of the primitive Earth by processes of subduction is still a matter of speculation. This review article is expected to arouse curiosity to pursue further research studies to unravel more novel information on the diverse activities, ubiquitous distribution and potential utilization of these organisms.