"If
surface habitats were short-term, that doesn't mean we should be
glum about prospects for life on Mars, but it says something about
what type of environment we might want to look in... The most
stable Mars habitats over long durations appear to have been in the
subsurface. On Earth, underground geothermal environments have
active ecosystems."
Source: Bethany Ehlmann, assistant professor at
the California Institute of Technology and scientist at NASA's Jet
Propulsion Laboratory in Pasadena, lead author of a study that
suggests that if life ever existed on Mars, the longest lasting
habitats were most likely below the Red Planet's surface.