This image from the HiRISE camera on the Mars Reconnaissance Orbiter, shows cracks in the rocks on Mars that once formed the underground plumbing through which groundwater traveled.
NASA / JPL-Caltech / U. Arizona / Chris H. Okubo
HiRISE image of possible former groundwater paths
Groundwater flow on Mars has been speculated for a long time, but it takes powerful cameras like HiRISE to actually find the evidence. These cracks resisted erosion because they were filled with minerals deposited by groundwater, so now we can see them as positive relief.
"This study provides a picture of not just surface water erosion, but true groundwater effects widely distributed over the planet," said Suzanne Smrekar, deputy project scientist for the Mars Reconnaissance Orbiter at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Groundwater movement has important implications for how the temperature and chemistry of the crust have changed over time, which in turn affects the potential for habitats for past life."
Too bad we didn't find these a year ago so they could be considered as an MSL landing site...