Since the discovery of the subseafloor biosphere, life is shown to extend deeper into the Earth's subsurface. Initially, the deep subsurface biosphere was considered to be the realm of prokaryotic organisms, because of the constraints of temperature, energy, dioxygen and space, which seemed to preclude the possibility of more complex, multicellular eukaryotic organisms from surviving at deep depths. However, the more we study the deep biosphere, the more it becomes apparent that prokaryotes are not the only organisms adapted to these extreme conditions.