The Earth is the miraculous body, where its planetary conditions fit the conditions for life and the Sun's mercy gives rise to the prosperity of life. The eat-eaten connection of life is known as food chain, and almost all of the food chains on Earth start from photosynthesis, and photosynthesis depends in turn on the radiation from the Sun. In this sense, most of the Earth's organisms are Sun-eaters ultimately. However, recent studies proposed the significance of the lives independent of photosynthesis such as the ones in deep-sea and deep subsurface. Findings of the life in the dark facilitate the recognition of Earth-eaters, and the idea that deep subsurface of the Earth (and possibly other planetary bodies) serves as the cradle and nursery for life.
The idea of deep subsurface life is not necessarily new, however, it is since 1980's that the idea has been scientifically hypothesized and tested. By now, the existence of deep subsurface biosphere is scientifically recognized, and current interests are placed on: the abundance (biomass), diversity, biological limit (potentials), involvement in biogeochemical cyclings, and association with the origin(s) of life. Even astrobiological curiosity relates the ubiquity of "life in the dark" in deep subsurface to the presumed presence of extraterrestial life. This communication provides the geo-biological background of the deep subsurface biosphere and reviews some important topics from recent studies.