Scientists have now embarked on a census to catalogue the life buried beneath the earth's surface - to help them understand the origins of life on earth or reveal life that could survive on other planets.
To unlock the mystery, Sharon Grim of Marine Biological Laboratory in Massachusetts, and Rick Colwell, a microbiologist at Oregon State University, began analysing genetic data from all the underground archaea and bacteria they can, including a key identifying set of genes.
"Though the results are still early, they are finding that the life at that depth is incredibly diverse," Colwell said.
"We have found one type of archaea in about a third of their samples from all over the world, and in all the archaeal communities sequenced. It may be a keystone species that needs to be present for such primitive organisms to thrive," Colwell added in a report that appeared in LiveScience.
"The dark-dwellers reproduce only every few months or years and have slow metabolisms, with some organisms moving the equivalent of just a few electrons per second," Jens Kallmeyer, a geochemist at the University of Potsdam in Germany, was quoted as saying.
"We cannot understand how an organism can possibly survive on that little energy," Kallmeyer added.
Since these creatures are living with strange and scarce energy sources, little to no water and scorching heat, the research can throw a great insight about life on other planets.