The Subterranean realm includes the earth’s crust and subsurface voids characterised by an absence or very low intensity of sunlight.

Energy is therefore a major resource driver in these ecosystems, although other resources such as water and carbon, may sometimes be limiting. The ambient environment is relatively stable, although extreme temperatures occur in local areas influenced by vulcanism and substrate type influences availability of nutrients and seepage of moisture. Disturbances are rare and limited to mass movement or subterranean fires restricted to coal, oil or gas strata, with combustion rates limited by oxygen. Subterranean trophic webs are truncated, lacking photoautotrophs, herbivores and large predators. The microbial biota is a key feature in the expression of different subterranean ecosystems.

The subterranean realm transitions to subterranean freshwater and subterranean tidal systems.