They are referred to as intraterrestrials, organisms that live inside the Earth. Most live beneath the bottom of the oceans. Some live in the tens of meters of mud just beneath the seafloors; others, following fractures in rock, live hundreds of meters down. By some estimates, as much as one-third of the planet’s biomass (the weight of all its living organisms) is "buried" beneath the ocean floor.
By comparison to the nutrient rich coastal zones, many of the communities on the ocean bottom live in nutrient poor regions. In nutrient rich areas oxygen is frequently only found in the upper 1-2 cm of mud; any deeper and it is consumed. But in some areas where the waters are nutrient poor, oxygen penetrates up to 80 meters of sediment. One thesis is that the microbes use oxygen slowly, so more is available to penetrate. Another possibility is that the microbes have a separate, unusual source of oxygen, natural radioactivity. Radioactive decay creates particles that can split H2O into hydrogen and oxygen (the process is known as radiolysis) [for a rough overview of radiolysis, see http://en.wikipedia.org/wiki/Radiolysis]. In this admittedly "exotic" thesis, microbes consume these elements.
It is also important to bear in mind that water does not merely sit on top of the seafloor, but in fact moves through the undersea arena, cycling the equivalent of the ocean’s entire volume through the crust every half-million years or so. Recent research has identified one of these flows. At Juan de Fuca (see http://en.wikipedia.org/wiki/Juan_de_Fuca_Plate) researchers have found two volcanoes, 50 kilometers apart. Cracks connect the two. Water goes in at one volcano and comes out at the other. Most of the fractures in the ocean crust here run north to south, making that the probable direction in which microbes also move. The cracks serve as a sort of microbial superhighway, allowing the microbes to flow along easily, carried by water. Thus, when thinking of subseafloor microbial populations, it is not correct to think of them as necessarily staying in one area. Because of differences in nutrient flows from the surface and about the seafloor, because of cracks, and because of water flows, among other traits, the seafloor has a variety of different environments.
As one would anticipate with the presence of various environments, the deep-sea microbes also express diversity. Researchers have found not only broad classes of bacteria, but also archaea [see http://en.wikipedia.org/wiki/Archaea]), fungi, viruses, and other types of organisms.
Studies, reports, and websites on the existence of life below the seafloor can be found at: http://www.nature.com/ismej/journal/v5/n4/abs/ismej2010157a.html; http://publications.iodp.org/preliminary_report/329/index.html; http://www.nature.com/nrmicro/journal/v9/n10/abs/nrmicro2647.html; http://www.darkenergybiosphere.org/; http://www.sciencemag.org/content/320/5879/1046.abstract; http://geology.gsapubs.org/content/early/2011/02/03/G31598.1.abstract; & http://www.ncbi.nlm.nih.gov/pubmed/18509444.