http://earthsky.org/space/mars-express-spacecraft-finds-evidence-of-subsurface-lake-on-mars

There is still some reason for caution, as the results haven’t been separately confirmed by another orbiter yet, but the evidence from Mars Express is certainly compelling. Some researchers also suggest the possibility that the lake may actually be a region of very damp soil or “muddy sediments” instead of actual liquid water, but only further study will determine which is the case. NASA’s Mars Reconnaissance Orbiter (MRO) has also looked at the same region with its radar, called SHARAD, but hasn’t detected the lake. However, this is thought to likely be because its radar is tuned to different wavelengths than the one on Mars Express, which are scattered by the polar ice before they reach the depth of the lake.

For now at least, it appears that the first underground Martian lake has been found. It’s also likely that if this lake exists, then there are probably others, as well.