Up to half of all life on Earth consists of simple microorganisms hidden in rocks beneath the surface. Scientists have suggested that the same may be true for Mars. When meteorites strike the surface of Mars, they act like natural probes, bringing up rocks from far beneath the surface.
Recent research has shown that many of the rocks brought up from the Martian subsurface contain clays and minerals whose chemical make-up has been altered by water, an essential element to support life. Some deep craters on Mars also acted as basins where groundwater likely emerged to produce lakes. McLaughlin Crater contains clay and carbonate minerals formed in an ancient lake on Mars. The fluids that formed these minerals could carry clues to as to whether the subsurface contained life.