McLaughlin Crater is 90.92 km (56.50 mi) in diameter and 2.2 km (1.4 mi) deep with a floor that is well below Martian “sealevel” and contains clays that bear iron and magnesium as well as carbonate. By the time eukaryotic life or photosynthesis evolved on Earth, the martian surface had become extremely inhospitable, but the subsurface of Mars could potentially have contained a vast microbial biosphere. Crustal fluids may have welled up from the subsurface to alter and cement surface sediments, potentially preserving clues to subsurface habitability. Many ancient, deep basins lack evidence for groundwater activity. However, McLaughlin Crater, one of the deepest craters on Mars, contains evidence for Mg–Fe-bearing clays and carbonates that probably formed in an alkaline, groundwater-fed lacustrine setting. This environment strongly contrasts with the acidic, water-limited environments implied by the presence of sulphate deposits that have previously been suggested to form owing to groundwater upwelling. Deposits formed as a result of groundwater upwelling on Mars, such as those in McLaughlin Crater, could preserve critical evidence of a deep biosphere on Mars. Scientists suggest that groundwater upwelling on Mars may have occurred sporadically on local scales, rather than at regional or global scales. “This environment strongly contrasts with the acidic, water-limited environments implied by the presence of sulphate deposits that have previously been suggested to form owing to groundwater upwelling.” Water-made channels which are now dry, appear to flow down the walls of McLaughlin Crater and stop well above the crater floor, which indicates they once provided water to a lake. “The deposits in McLaughlin Crater could have very high preservation potential for organic materials, in much the same manner as turbidites do on Earth.” Cyanobacteria, which are common in alkaline lakes on Earth may have aided in the formation of carbonate minerals in lakes such as the McLAughlin Crater on Mars. Sometimes these bacteria become form microscopic fossils. If similar conditions existed in the craters ancient alkaline lake fossils of micro-organisms may still be there awaiting us.