This story was news a week or so ago but the image above taken by the Mars Reconnaissance Orbiter, (and posted over at Astronomy Picture of the Day), is worth staring at for a while even if you have seen it before.
See those dark streaks running down the canyon walls of Newton Crater just south of the Martian equator? The streaks come and go with the Martian seasons. Weather gets warm (a few degrees above zero C is warm for Mars) and the streaks appear. Weather gets cold, they disappear.
Of course, because the Martian atmosphere is so thin the water, (if that is what it is), evaporates quickly for any individual outflow from the subsurface reservoirs (if that is what they are).
As a whole, however, the "streaks in spring/summer" vs. "no streaks in winter" paints a convincing portrait of a world with, perhaps, significant amounts of subsurface liquid water.
More than enough for future human colonists?