In 2008, as part of a feasibility study for radioactive waste disposal in deep geologicalformations, the French National Radioactive Waste Management Agency (ANDRA) drilled severalboreholes in the transposition zone in order to define the potential variations in the properties ofthe Callovo–Oxfordian claystone formation. This consisted of a rare opportunity to investigatethe deep continental biosphere that is still poorly known. Four rock cores, from 1709, 1804, 1865,and 1935 m below land surface, were collected from Lower and Middle Triassic formations in theParis Basin (France) to investigate their microbial and geochemical composition. Rock leachatesshowed high salinities ranging from 100 to 365 g·L−1 NaCl, current temperatures averaging 65 ◦C,no detectable organic matter, and very fine porosity. Microbial composition was studied using adual cultural and molecular approach. While the broad-spectrum cultural media that was used toactivate microbial communities was unsuccessful, the genetic investigation of the dominant 16SrRNA gene sequences revealed eight bacterial genera considered as truly indigenous to the Triassiccores. Retrieved taxa were affiliated to aerobic and facultative anaerobic taxon, mostly unknownto grow in very saline media, except for one taxon related to Halomonas. They included Firmicutesand α-, β-, and γ-Proteobacteria members that are known from many subsurface environments anddeep terrestrial and marine ecosystems. As suggested by geochemical analyses of rocks and rockleachates, part of the indigenous bacterial community may originate from a cold paleo-recharge ofthe Trias aquifer with water originating from ice melting. Thus, retrieved DNA would be fossil DNA.As previously put forward to explain the lack of evidence of microbial life in deep sandstone, anotherhypothesis is a possible paleo-sterilisation that is based on the poly-extremophilic character of theconfined Triassic sandstones, which present high salinity and temperature.< Réduire