Most of the earth's microbial biomass is hidden in the subsurface. According to estimates, microorganisms can be found at depth of up to five kilometres below the continental surface. Here they also colonize solid rock. Since this deep biosphere is difficult to access, researchers know little about the composition and role of these microorganisms in biogeochemical cycles. A research team from Friedrich Schiller University Jena and its Cluster of Excellence "Balance of the Microverse" as well as other research networks and institutes have now discovered that limestone serves as an archive for microbial colonisation of the subsurface.