We have no idea whether there’s microscopic life on Mars. Any hypothetical Martian microbes would probably live under the planet’s dry and frozen surface, and nothing we’ve sent to Mars so far can dig to any significant depth.
But this is about to change. We’ve already discussed the European Space Agency's ExoMars rover, which will arrive on Mars in January 2019 and drill two meters (about six and a half feet) into the surface in search of life. Now ExploreMars, a U.S. nonprofit organization dedicated to, well, exploring Mars, is working on a new crowdfunded ExoLance initiative that would send a series of two-meter “bunker buster” drills to stab into the Martian surface and report back on their findings, at considerably less cost—and, subsequently, over a much wider area—than ExoMars can manage.
ExoLance representatives discussed their plans in more detail back in April:
If there is life two meters or less below the Martian surface, odds are good that one of these initiatives—or their successors—will find it. But if Martian life exists deeper than two meters under the surface (which is possible, if not probable), they won’t. It will be important for everyone who supports this kind of research to keep public expectations reasonable, and to spread the word that the lack of visible evidence for Martian life on and near the surface tells us little about the possibility that something lives deeper below.