
Mars was likely, very long ago, blue from oceans of water that covered its surface.
But the now-red world may still hold liquid water underground, stowed deep and out of sight. If there is water down there, how would we find it?

Mars was likely, very long ago, blue from oceans of water that covered its surface.
But the now-red world may still hold liquid water underground, stowed deep and out of sight. If there is water down there, how would we find it?