Probes designed to find life on Mars do not drill deep enough to find the living cells that scientists believe may exist well below the surface of Mars. Although current drills may find essential tell-tale signs that life once existed on Mars, cellular life could not survive the radiation levels for long enough any closer to the surface of Mars than a few metres deep – beyond the reach of even state-of-the-art drills.
"It just isn’t plausible that dormant life is still surviving in the near-subsurface of Mars – within the first couple of metres below the surface – in the face of the ionizing radiation field. Finding life on Mars depends on liquid water surfacing on Mars, but the last time liquid water was widespread on Mars was billions of years ago. Even the hardiest cells we know of could not possibly survive the cosmic radiation levels near the surface of Mars for that long."
The best places to look for living cells on Mars would be within the ice at Elysium because the frozen sea is relatively recent – it is believed to have surfaced in the last five million years – and so has been exposed to radiation for a relatively short amount of time. link
Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology. 2007. L. R. Dartnell et al. GEOPHYSICAL RESEARCH LETTERS: 34