The oceanic crust makes up the largest potential microbial habitat on Earth. Yet next to nothing is known about the abundance, diversity and ecology of its biosphere. Because of difficulties with sampling, our understanding of the deep biosphere of subseafloor crust is, with a few exceptions, based on a fossil record. Paleontological material is, thus,
central in the exploration of the subseafloor biosphere. The fossil record shows that both prokaryotes and eukaryotes occur at these depths, and morphological diversity suggests symbiotic relationships between the organisms.
Frequent bio-etching into mineral substrates, further suggests that both prokaryotes and eukaryotes are powerful geobiological agents, and their activities might influence global element cycling. The subseafloor biosphere
represents some of the oldest signs of life on Earth, and the fossilization environments present a means to study the evolution of the deep biosphere in deep time. The oceanic crust may have functioned as a protected haven from
hostile surface conditions on the early Earth, and life may have originated and evolved at depths. A similar scenario is possible on other terrestrial planets despite large environmental differences.
