Temperature appears to be the primary factor in limiting how deep extremophiles can go. No known microorganism can live for long at 120 degrees Celsius. Since the surface temperature averages 15 degrees Celsius and the temperature in the ground increases with depth about 19 degrees Celsius every kilometer, extremophiles should die off between five and six kilometers below the surface of dry land. Near geothermal hotspots, subsurface temperatures rise much more rapidly, and life survives only near the surface. In deep ocean sediments, where kilometers of cold water above keep temperatures down, extremophiles may thrive at depths of several kilometers below the ocean floor.
Pressure limits the range of extremophiles less than temperature does.
Most microorganisms can survive pressure 600 times atmospheric pressure, which corresponds to a depth of six kilometers. At that depth in most locations the temperature likely exceeds 120 degrees Celsius. Lack of water and chemical nutrients likely prohibits deep subsurface life in arid, geologically stable regions. Conversely, extremophiles may be more abundant deep in active geological features, such as faults, mid-ocean ridges and salt deposits, where fluids and nutrients flow more freely. Subsurface environments are so austere some extremophiles live in a state of nearly suspended animation.