For obvious reason, scientists don't know a lot about these microbes, but researchers estimate they account for anywhere between 20 and 80 percent of Earth's bacterial and archaeal biomass.

Now, thanks a first-of-its-kind study conducted in an abandoned mine shaft, scientists have a better idea of how these hidden microbial communities are distributed beneath the planet's surface.

The new research, published Friday in the journal Frontiers in Microbiology, showed the mineralogical makeup of Earth's deep-lying rocks drives hotspots for subsurface life.

For the study, scientists cultivated microbial biofilms on rocks located a mile under Earth's surface inside South Dakota's Deep Mine Microbial Observatory, part of a former gold mine now known as the Sanford Underground Research Facility.

Using microscopy, spectroscopy and spatial modeling, scientists tracked and analyzed the growth of the experimental biofilms. The results showed the biofilms were thickest on rocks with iron-rich mineral grains.

"Our results demonstrate the strong spatial dependence of biofilm colonization on minerals in rock surfaces," study first author Caitlin Casar said in a press release.

"We think that this spatial dependence is due to microbes getting their energy from the minerals they colonize," said Casar, earth scientist and doctoral candidate at Northwestern University.

The study's authors' hypothesized that mineral composition drives the distribution and diversity of microbial communities deep underground, and the latest findings confirmed as much.

The findings should help scientists predict the locations of subsurface microbial hotspots, as well as better understand other subsurface phenomena.

"Our findings could inform the contribution of biofilms to global nutrient cycles, and also have astrobiological implications as these findings provide insight into biomass distributions in a Mars analog system," said Casar.